They have no recourse

They can do all of the following:

1. Report the seller to the platform for selling stolen goods.
2. Return the stolen goods to the rightful owner, if they’re able to get their information, or if not, to Valve or even just the police department.
3. File a suit in small claims court against the seller for damages (the amount they paid + any other expenses they’ve incurred, like the cost to mail it to the rightful owner, including legal fees) or, if the platform won’t provide the seller’s information, against the platform itself.