Well, obviously legally it's a problem, yes. But there's no way to know if the assets used for your game are acquired legally or not. The pseudonym you use to buy the assets can be different to the one used to publish the game, or you can lie by saying that you make the rendering yourself and pay someone that will have legally acquired the assets. So, nobody can know, and I doubt that someone will start an action in justice with just a suspicion that perhaps at least one of the assets haven't been legally acquired.
This said, it's also a moral problem. Yes, we are on a pirate site, but we don't try to earn some money with the games we pirates, which is different if you use the assets. So, starting with pirated assets, why not if you really lack of money. But buying them, one by one, once you've earned enough money seem, at least for me, the thing to do.
After all, the guys who made the assets generally are like you, someone with an idea and few capabilities, trying to earn some money with this. They deserves their share of the money you'll make.
