WHEN Marty McFly donned his self-lacing Nike trainers in the distant future of 2015, he really should have kept them in the box. Almost 30 years on from “Back to the Future II”, Nike’s real-life version, released in 2016, is the most expensive training shoe on the planet, with an average resale price of $32,275. These rarest of shoes (only 89 pairs were made) are at the apex of a resale market that has been carefully nurtured by Nike and other trainer titans since the late 1980s.

Every Saturday morning across America, queues of “sneakerheads” form outside trainer shops. Many are adding to their hundred-pair collections, but the rest are seeking shoes to sell in the secondary market. As brands try to strike a balance between generating instant revenue and restricting supply (which creates demand, and more revenue later), the secondary trade thrives. In America it is worth an estimated $1.5bn a year, a tenth of the trainer industry’s value.

By announcing small runs, without restocks, brands build huge excitement around specific lines. One man epitomises the importance of limited releases for the big firms. Since Kanye West switched allegiance from Nike to Adidas in 2013, Adidas’s share of the resale market has risen from less than 1% to 33%, and its share price has doubled. His latest shoe, around 100,000 pairs of which were released in April, is selling for more than double the retail price. The hype has not been without its problems. Black-Friday-style chaos in the shops has become so common that brands have taken steps to calm shoppers, instructing retailers to move trainer releases from midnight to early morning.

A combination of eBay, social media and online markets has helped move the secondary market off the street. EBay pioneered collectible-trainer resales online in the late 1990s and accounts for a third of the market, but is now less dominant, partly due to an epidemic of counterfeiting. Twitter and Instagram, where consumers scroll through feeds and interact directly with sellers, are the latest boosts to the secondary market. Annual sales directly through Instagram have been estimated at $200m in America.

“Stockmarkets” for box-fresh shoes have also sprung up, offering anti-counterfeiting services from professional sneakerheads. These function like traditional exchanges, with sellers setting a price and bidders making offers, except that when a sale takes place the trainers are shipped via companies for verification. StockX, founded in February 2016, hosts more than 100,000 trainer portfolios and expects sales volume to pass $100m this year. GOAT, a marketplace app started in 2015, claims 1.5m members.

There are big sums to be made. Josh Luber, co-founder of StockX, says that each trainer’s appearance on the markets is akin to an initial purchase offering: “The sneakers’ value is immediately realised and people make a lot of money from that initial pop.” A portfolio of ten popular pairs on StockX, all released in the past six months, would have yielded a return on investment of almost $7,500, or 280%. Time to go short?