Free tickets given to Swansea City supporters have appeared on ticket selling websites for more than £60.
The club said it was "extremely disappointed" as players had paid for 3,000 tickets to get as many as possible to the game at Sunderland on Saturday.
The free tickets had been snapped up within hours, officials said.
The Swans climbed out of the Premier League relegation zone last weekend with two games remaining.
Boss Paul Clement said after his team's crucial 1-0 win over Everton on Saturday that it was a "shame we can't bring all the fans" to Sunderland.
It is not known exactly how many tickets have been put on sale but the club said it was not more than a "few dozen".
A Swansea City spokesman said some "genuine fans" had missed out on free allocation.
"While we would warn supporters that there is always a risk with purchasing tickets on these sites, anyone found doing this risks having their season tickets and Jack Army membership revoked," he added.