Problems with the buyer

I sold the keys to the game.
The man bought the game and did not confirm the deal.
When the automatic closing of the transaction left 40 minutes - he opens the dispute and says that the key does not work.

Another buyer says that he" resells the keys " and said that his client received a non-working product. How do I know his buyer isn’t cheating on me? Or maybe he buys and sells gameflipm on g2a. And he says that the key doesn’t work. However, he gets the money.

I don’t know what to do. Of the 20 deals, I only have two with problems. How to solve this problem?

Can you please send me via PM the information about this case (such as the transaction ID) so I can properly help you?