When a 1949 Chieftain armchair by Danish designer Finn Juhl sold last fall for $780,000 at auction, mid-century furniture dealers yawned. Mid-century modern, a functional, clean-lined movement in furniture spanning the two decades after the Second World War, has rocketed in value over the past 10 years – in line with top salaries, real estate and design savvy. Chairs, the items for which many mid-century designers flexed their most creative muscle, are the "iconic" items of the era. And they've become apt market barometers.

As demand grows from middle America to the Middle East, stock is disappearing, and dealers who might have charged $1,000 10 years ago for a Soft-Pad chair by Charles and Ray Eames today charge twice that. Greg James, a buyer for the Fabulous Find in Victoria, reckons prices have risen "30 per cent across the board over the past five years." Less ubiquitous chairs from voguish names such as Poul Kjaerholm, a Danish minimalist, have appreciated even more. "A decade ago, I'd sell Kjaerholm's PK22 chairs for £700 [$1,293]. Now it's more like £2,500 [$4,617]," says Rob McClymont of the Modern Warehouse in London. "What used to be 'second-hand' is now 'modern antique.'"

Fans of the look see it as functional art or fine, collectible design. "When you buy original mid-century modern, your money is not gone," says Lawrence Blairs, owner of Atomic Design in Toronto. "It's merely locked away."

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If you want to get in on the ground floor, follow the rules of the pros.

Decide what you want

"Always buy what you like and not what you think will be a good investment," says Petra Curtis, co-founder of the Midcentury Modern Marketplace directory. "You have to connect with the piece in some way," says Blairs. "Maybe it reminds you of your grandparents' house or the colours in a painting. Even if you have limited knowledge of the designer, it's ultimately how it makes you feel that's important."

If it's Eames, you'll be in the majority, but you'll also need to keep on the lookout for knock-offs, which have become prevalent in recent years and led to prices levelling off.

"Teak chairs can't be copied officially," says James, meaning their value will hold. James recommends lesser-known designers, such as Peter Hvidt of the Danish studio Hvidt & Mølgaard. "Everything he did was solid teak and finger jointed – awesome stuff."

Vintage, reissue or knock-off?

You get what you pay for with a knock-off: inferior quality and dubious materials. "You'd be lucky to sell it for $50 the next day," says James, "and it'll eventually end up as landfill."

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Reissued classics by manufacturers such as Fritz Hansen and Vitra are handy if you're after, say, six identical Hans Wegner Wishbone chairs for your dining room, at $1,000 a pop. But take heed: "Once they leave the showroom, they'll lose 30 per cent of their value, like a car," says McClymont.

A mint-condition vintage Wishbone, by contrast, might cost $500 and will appreciate. "The only time I'd advise someone to buy new is when the piece is rare, like Juhl's NV45 chairs, which you don't find any more. You could wait years for a pair, and when they go to auction, they sell for ridiculous money."

Choose your dealer carefully

The Internet is invaluable for shopping, but if you're serious about buying, use it as a means to an end. Many online portals are staffed at best by rookies, who don't know an Arne Jacobsen from the generic, and at worst by cowboys. The veteran portal 1st Dibs is an exception: trustworthy, albeit expensive. It monitors potential merchants for years before allowing them to sell through the site, "so you know you're going to get quality," says Israel Jones, owner of the Swanky Abode in Ohio.

Enthusiasts should frequent vintage-furniture fairs and storefront dealers. "Build a rapport," Curtis says. "Dealers love to talk the nuts and bolts off a piece. You won't get the same ground knowledge from a sales assistant." Suss out the dealers in your area. Old hands get great deals down the line, from sourcing to restoration – and that translates into competitive pricing. (Just to make sure, search online for price comparisons.)

Crucially, well-known dealers get right of first refusal when locals sell off their contents. "People in their 80s walk into the shop with pictures and ask, 'Do you want this stuff?'" says James. "And that's the best stuff, because if they're between 80 and 90, they bought it in the fifties and early sixties."

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It goes both ways. If you can get on the waiting lists of your favourite dealers, they'll come to you with the best merch. "A quarter of the stuff we sell you don't even see on our website, because it goes to the top of the wish list," says James.

Identify quality

Once you've zeroed in on a find, sit on it. Wiggle it. Assess if the shock mounts are firmly attached. Check for corner brackets, used by amateurs to repair broken teak – they'll drag down the value. Take into account whether the whole chair is original or a marriage of disparate parts. Ask about provenance.

Then Google the name for marks of authenticity. For example, Eames DSW chairs were made of Fiberglas while reissues are inferior plastic. Early models manufactured by Zenith have a checkerboard label on the underside; models by Cincinatti Milacron have a crescent "C"; Summits Plastic models have a trademark as well.

Danish teak of the early postwar period has a more interesting grain than the farmed teak of later stock. "And if it's upholstered," says Israel Jones, "research whether the upholstery is genuine or a replacement."

Spot an opportunity to save

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Dealers don't tend to have sales – unless they want a big turnout at a furniture fair. "When they're selling at the Midcentury Modern Show in London," says Curtis, "they offer a special one-day discount of 10 per cent."

There's a lot of leeway on the hunt for a cheap Eames chair. Later models, or pieces with replacement legs, can go for well under $1,000. And in the Eames world, orange is the new black – in the 1950s, orange chairs were mass-produced whereas seafoam and grey are rare – and more expensive(think $1,300). By contrast an early-1950s, mint-condition, rope-edge Eames rocker will put you back $3,700.

Prices do vary across regions, however. In big retirement destinations, where people tend to sell off their collections, stock is plentiful so prices come down. "Palm Springs and Florida are good places to look," says Curtis. The Canadian equivalent is Victoria, where a cluster of Danish-modern stores in the 1950s and '60s sold to young families. "Now, when those same people downsize into apartments," says James, "they give us a call."