Over the last two decades, soccer has evolved from strictly sport to, Cashmore said, a division of the entertainment industry. Players like Beckham, Lionel Messi, Cristiano Ronaldo, Wayne Rooney and Ronaldinho are worldwide brands as much as they are soccer stars.

Thus, some jerseys can be more than keepsakes — they can be commodities. The Brazil jersey worn by Pelé in the 1970 World Cup was sold in 2002 by the family of the Italian player he had traded with for $310,000.

Nigel Spill, a sports memorabilia dealer in Los Angeles, said a game-worn jersey of a player like Messi, the reigning world player of the year, would be comparable to what a Derek Jeter jersey would fetch, easily into five figures. “It’s not an innocent jersey swap anymore,” Spill said.

Indeed, when Nadir Haroub of Tanzania received what could be a valuable jersey when he traded with the Cameroon star Samuel Eto’o after a World Cup qualifier in 2008, the Tanzania Football Federation threatened to force Haroub to pay for a replacement jersey because it could not afford one. Eventually, the federation relented.

When the Los Angeles Galaxy played Barcelona and A. C. Milan in exhibitions last summer, many of the Galaxy’s younger and less-well-compensated players schemed about trying to get the jerseys of Ronaldinho and Messi. Similar conversations took place among players on the United States team in South Africa, the American left back Jonathan Bornstein said.

Still, there is an etiquette to the jersey swap. Typically, players who trade either know each other — perhaps they are club teammates — have battled over the same area of turf or just happen to be near each other when the game ends. Even those who are eager to snag a particular shirt try to be discreet.

To Bornstein, all the accounting that takes place in shirt swaps also affirms where a player fits in the increasingly global game. “There’s definitely a totem pole of status,” Bornstein said. “It lets you know where you are on the totem pole.”