It doesn't make any sense for him to ask why you're asking him for a refund, because you are fully entitled to one if the replacement key doesn't even work.

He's still advertising his key sales, just on a different thread. There is also another outstanding accusation from the user politehalo that basically had the same experience as you with the same person. It certainly isn't looking good for your chances of getting your funds back at the moment.

But this is why selly.gg sellers shouldn't be trusted. Selly.gg and other autobuy sites give off the facade that you're protected through some escrow system when in reality you're literally sending the money first without any guarantees. In the future, always deal with escrow when the party you're dealing with isn't trusted. Even a free escrow would do.