The temperature drops perceptibly as Doug Bierton discreetly sweeps the bracelet on his left wrist over a wall-mounted sensor and enters the vault.

Like access to this locked room, the climate inside it is strictly controlled. To the uninitiated, this might seem excessive. Its contents are not jewels, or watches, or the treasures of some lost civilization. They are, instead, soccer jerseys: roughly 6,500 of them, hanging from great metal racks.

This is just a tiny fraction of the stock that Mr. Bierton’s company, Classic Football Shirts, holds at any given time. In the cavernous warehouse that surrounds the vault, there are at any given moment more than a million jerseys, hats and other pieces of apparel. This room is for those items Mr. Bierton cannot bear to part with, the pieces — including match-worn editions of some of the most iconic jerseys in history — he has deemed too precious to sell.

At first glance, it does not seem like that. The overall impression is of an overstuffed closet: a jumble of man-made fabrics and a riot of color, all vivid reds, lurid yellows and royal blues.

Mr. Bierton, though, can find any particular jersey in just a few seconds. He has memorized the provenance, the background and the story of each one. He knows the stains, and the defects, that prove their authenticity.

He has been in the business of buying and selling soccer jerseys for almost 20 years, ever since he founded Classic Football Shirts, an online platform for trading vintage editions, with a friend in 2006. At heart, though, he remains an enthusiast, a collector and a fan.