Unpaid players aren't allowed to make a bit extra on the side.
In the NCAA, where players don't get paid and networks make almost a billion dollars in revenue, players are now being investigated for selling their team-issue Air Jordans. It began with North Carolina. The school's football program turned itself in to the NCAA, after finding out that 13 of their players sold sneakers that were given to them as part of a team sponsorship deal. The "retailer," as ESPN calls it, told UNC that it had bought shoes from players at Michigan, Marquette and California as well. The UNC players who sold their Jordans have been suspended for between two and four games.
The man for whom Jordan Brand is named, Michael Jordan, played college basketball for the Tar Heels, and so their Jordan sponsorship is, to say the least, a special one (not to mention that baby blue). So special, in fact, that even Virgil Abloh, when reimagining the Jordan I, chose to do a UNC version, which often goes for over $1000. Now imagine the price of a shoe custom designed in a limited run of 50 pairs for the real team. According to ESPN, they can sell for more than $3500, which is why Jordan–seeing the potential–has released many school-specific shoes to the public, like the Michigan 12.
The investigation and suspension of players who sell their shoes seem strange. NCAA players (basketball, football or whatever) are not allowed to be paid. When some of them were paid under the table by coaches, their teams weren't selected for March Madness. The schools and TV networks who broadcast the games make a fortune (even coaches get paid) while the players, some of the most elite athletes on earth, do not get a salary.