May I use your tileset in a game that I am creating as a commissioned project?
Viewing post in PCFW: Magic Days & Scary Nights Tileset comments
If you just hand over the finished (and encrypted) game, I see that as nothing different than creating a game to sell on a marketplace. That would be fine.
I'm not sure about open files the client will be editing, as they'd be actively using the tileset and obtained a copy of it they did not pay for, so they'd probably need to buy it too.