Spillers Records in Cardiff has served music lovers for nearly 120 years, but competition from the internet makes its future far from certain. We spent a day there meeting staff and customers

The light-up sign in the window of Spillers Records, a suitcase-sized chock of blue and white acrylic, is off. The doors are locked. Outside, a man wearing a woolly hat and headphones checks the time: 20 past. He checks again. Still that.

Spillers opens at 9.30am, and it may be to divert the over-eager morning customer that some window space has been turned into a small exhibit, celebrating the shop's rich history in Cardiff. There are record sleeves from its days as a "phono exchange", run by the Spiller family from 1894 to the 1940s. There are 7-inch single bags – relics, like the light-up sign, of the 1970s, when the shop was run by a local man called Nick Todd. A cardboard replica of Tom Jones invokes history more recent, the cut-out draped in branded Spillers T-shirts that were must-buys for loyal customers when the shop faced closure in 2010.

It has been a difficult decade for record shops like this. Streaming, downloading, home shopping, pirating – the internet-inspired woes of the music industry have been disruptive from the perk-denied pop star down, but effects at the retail end have been particularly severe. Robust-seeming chains have folded, and of the 900-odd independent record shops that were in business in the UK six years ago, fewer than 300 are trading now. The downturn was part of the spur behind International Record Store Day (21 April), an event thought up in 2007 and officially launched the following year to say, essentially, support these places now or one-click-buy on Amazon forever more.

With the fifth annual Record Store Day approaching, Spillers stands as the world's oldest surviving record shop. It opened more than 100 years ago, when the wax cylinder was king, and continues today, under the anarchic reign of the MP3. What is Spillers still doing here? What's it like to work here, to shop here? To own it, to rely on it?

Today is a Friday in mid-March, and the morning's first boxes of stock have been delivered. It is (finally, the man in headphones must be thinking) 9.30am. Manager Ashli Todd arrives to open up.

First order of business, she says, flicking on lights: "We need to put on some tunes or we're not a record shop." Steve Taylor, a Spillers employee for more than 20 years, obliges with some gentle folk. "Something to ease us into the day," he says, hunkering down on a stool behind the wall-to-wall counter.

Around him are towering shelves, packed to capacity with plastic-wrapped albums. On the other side of the counter, in the customer half of the shop, racks are filled with photocopied replicas of these albums – a measure to reduce the effects, if not the actual incidents, of theft. Last week a dim-witted shoplifter made off with a handful of sleeves from the world music section, all dummies.

Todd explains some of the shop's arcane methods, pointing out the wooden pigeon-hole where albums that need to be reordered go, and the fact that every shelved CD has a little square of card folded around it on which there's a handwritten sales history. "It's a system I inherited," shrugs Todd, but it seems to work. She plucks a record from the nearest shelf, an album by defunct American rock band the Flamin' Groovies, and sees from the Biro-y scribbles that it last sold in early 2005. So that'll need to come forward on the customer racks…

Todd is 28, slight and pretty, her red hair dramatically shorn. Taylor, 43, is bigger, bearish, with shoulder-length black locks. Hurrying in at a quarter to 10, Graf Middleton, 30, is Spillers' only other full-time employee. Tall, bearded, wearing fingerless gloves, he gasps: "I live walking-distance away. Jogging-distance, if I'm late."

Teas are prepared by Middleton in penance, and after that he starts scissoring open delivery boxes. One contains copies of an album by a Cardiff band called Y Niwl. Not a lot of people have heard of them, says Middleton, describing their sound as "Snowdonian instrumentalist surf-guitar". But the Spillers gang have been actively promoting Y Niwl, who record locally, giving the album a lot of play time in-store. It's become a decent seller. "Every indie shop will have its version of Y Niwl," says Todd, because if customers are making the effort to come in, it's important to be able to recommend something they can't easily get elsewhere.

Recommendation is central at Spillers. Staff like to tell customers what they should listen to, and like to be told in return what they should stock. Every Christmas, employees and regulars write up "best of" lists for the year, to be tacked on the walls alongside the band posters and mounted record sleeves. Today there's just one on display. Samuel, aged eight (and the son of a regular), quite liked Ron Sexsmith and Canadian band Destroyer in 2011. "Music is sharing," says Todd.

A customer at the far end of the shop confirms this. "I'm 47 years old," says Graham from Caerphilly, "and thanks to this lot I'm suddenly a fan of a musician called Captain Beefheart. I'll probably get another of his albums today." In fact, Graham approaches the counter and asks Todd about Welsh musical stalwart Euros Childs. Todd admits they don't have any copies of his latest but (typing furiously) she places an order. Actually she's emailing Childs directly, and the musician replies to say he'll come by with copies in the afternoon.

As well as managing the shop, Todd co-owns Spillers with her sister, Grace, who works at a local museum and runs the shop's accounts out of hours. The siblings bought the shop two years ago from their dad, Nick, after a complicated period in Spillers' history. For decades the shop occupied a different site around the corner, on a busy thoroughfare called the Hayes. In the 80s and 90s Spillers was packed, Todd says, not only with regular customers and passing shoppers but with sales reps from distributors around the country. "A constant queue of them, like a doctor's waiting room," she recalls. Such was the health of the record industry.

When the industry began its millennial downturn, this coincided with more local problems for Spillers. New landlords tripled the rent. An expensive refurbishment of the Hayes meant that for months the shop had to trade from the middle of a building site, construction dust fogging the windows. Nick Todd wanted to sell and for a time it looked certain Spillers would close… Then his daughters proposed they take over. They'd both worked in the shop growing up, and thought they could make it work on a new site, in a maze-like retail arcade that abuts the Hayes.

"Down the winding passage," says Todd, speaking to a customer on the phone. "Yeah, keep going. Keep going. Keep going." Shortly after the move in 2010, descendants of Henry Spiller, the shop's original owner, came by to give the new venue the thumbs-up. Some long-standing customers, however, have grumbled. "You'd think we'd relocated to the moon," says Todd. As a sop to regulars, she's tried to let a layer of dust collect wherever possible. "…And speaking of regulars!"

A middle-aged man has taken up position by the counter, feet spread as if he means to linger. "I'm in every day," confirms Neil Gibbs, an accountant at a nearby bank. "Feels like it," retorts Middleton. An intense conversation about the guitar work of Smashing Pumpkins starts up.

It's the chat, Gibbs says later, that makes Spillers special. "The only conversation you get in the chains is 'Put your pin number in.'" Middleton, in fact, used to work for Zavvi (before it shut), also Virgin Megastore (before it became Zavvi), and though he's a little sensitive about a CV that marks him as a sort of record-store angel of death, he confirms that big-shop employees are not encouraged to engage in 20-minute conversations with the customers.

Lunchtime passes, and the phone rings again. "Just had six boxes that came in," says Todd. "I'd be really surprised if purple wasn't in there… A large in purple? No worries. And will you be exchanging the medium?"

She descends to the basement where the boxes of Spillers merchandise are kept. The T-shirts used to be worn by staff, she says, but "customers took a liking to them" so Spillers started selling them at £12.99 a pop. "Business is business. We sell Spillers tote bags, mugs, even Babygros. Not because we're crude but because the customers are there for it, and we control the mark-up. When we need to slash the price on an album to have a chance of competing with Amazon, this stuff " – she waves a purple T-shirt, large – "stops us going under."

Upstairs, sneaking an early-afternoon cigarette outside the shop, Todd meets a man in chunky glasses. He is called Chill ("or Richard, if you're my mother") and he runs a local music venue. Todd takes a look at the sale-or-return gig tickets he's brought her, like the T-shirts an important business seam for Spillers. Also a handy way to spark sales – shortly after 2pm, a student comes in to get tickets for a Wild Beasts show and goes home, too, with a Tom Jones vinyl that was temptingly positioned beside the counter.

Later, two more students, Lauren and Alex, both 19, browse the shop's racks. No, they say (fairly affronted by the age-based assumption) they don't prefer to get their music in MP3 format. "Because you want to come away with a thing for your money," says Lauren. Broadly, they say, they're fans of digitised music. Streaming on Spotify, for instance, makes it easier to unearth new artists, or to ensure you like their work before buying.

Taylor, preparing a new round of teas nearby, doesn't think download or streaming services are a particular cause of record-shop ruin. "For us it's more of a problem that online sellers can bring in stock on the cheap through the Channel Islands." On the shop wall someone has put up a sign that says, "We pay VAT", and Todd explains the reference to an obscure loophole that allows wily online retailers such as Amazon to undercut competitors by importing goods through Jersey and Guernsey. Happily, Todd has today seen a news story suggesting the loophole will soon close.

No time for celebration – it's 3pm, and there's an afternoon rush. The team are joined by a part-time employee, a 21-year-old with wild curly hair called Owain Lewis, to help serve customers. Suddenly the shop is packed. "There's no consistency," says Todd. "We can have a really shit Tuesday, a massive Friday… The unpredictability makes staffing a nightmare." Yet her staff seem very happy. Middleton is talking to someone about the red-vinyl version of an album he likes. Lewis has just sold an obscure record by a Welsh dub-reggae band for a whopping £35. "It's a brilliant job," says Taylor. "But unless you love music and love turning other people on to music, you couldn't do it."

Not long before closing, a regular called Mr Spencer comes in. An ex-RAF man, Mr Spencer has been frequenting Spillers, he says, since 1971. He takes a small notebook from his pocket to go over his purchase needs for the week, and as records pile up on the counter he speaks quietly about his passion for the shop. "In the RAF I was away all the time but I'd look forward to coming back – back here. Spillers is unique. It's… it's…" Unable to describe what it is, precisely, he bites his teeth together. How would he feel if Spillers closed? "Devastated," he says softly. "Devastated."

Trading hours wind away. Euros Childs drops off those promised albums. Incredibly, a man nobody has ever served before buys the repositioned Flamin' Groovies album, notching its first Spillers sale in seven years. At closing time it takes 45 minutes for Todd to urge her customers out.

"It's bloody hard to keep a place like this going," she'd said earlier. "If it goes tits up? We're not idiots. We won't sacrifice our lives just to keep the shop going forever. If it doesn't work out it doesn't work out. We'll give it a go while we can, and be really proud that we tried. But I love it. I love it."

It is 6.30pm. She shuts Spillers for the day.