Assuming that’s a different site, there is not much you can do. You can join that site and report that member. You can take a look at DMCA claims, but I’m not very familiar with them, and not sure if it can be applied here.
From my experience, a lot of people purchase steam games and try to “promote” them on different sites, or even pirate them.
To prevent this in the future, you can look at ways to “watermark” your game, so your name is clearly visible inside it, preferably in a way that can’t be removed. This won’t stop people from trying to steal it and promote it as their own, but it will make it clear that it’s yours. Another option would be to look at DRMs, but that has its own pitfalls.
