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Ouch! They don't have any system to let us know who purchased the game, as far I know… -_-'

I don't know anything about selling on steam but I bet you could put a menu option in your game that ties in with the Steam API and requests an itch key from your own server.

That's a good idea, but I removed my games from Steam, I don't know if their API still works for them.

Also, I have a RPG Maker 2003 game that doesn't have support to the Steam API anyway. =/