Does Facebook Marketplace have a way to report a bad seller or a transaction where you didn't get the product you were sold?
Unfortunately, this is going to be a hard learning experience. There's no way to buy a Steam game in "used" condition. For anything sold on Steam, it's sold as a license to the original buyer and that buyer cannot sell or transfer it to someone else. Same is true for Xbox and PlayStation games sold online and not as DVD discs.
This transaction could have been legitimate if the activation code had never been used. When you buy a new Steam game in a box, what you get inside is an activation code. But once the code is used and assigned to someone, the code can never be used again.
