And that impacts my gameplay how exactly? Zero, zilch, nada, nothing. You'd be completely stupid to use sites like that with the amount of money and cars the game throws at you for just playing the game. It's a complete non-issue. And just because a minority might abuse it, the whole thing should be discarded? This is the whole "there should be no livery editor because someone might draw a penis" conversation all over again (which was also a non-issue).
Fact is you can get lots of nice cars via the auction house, and often at a very cheap price. Does bidsniping happen? Yeah, sometimes it does. Are cars sometimes listed for a higher value than the dealership? Sure. But that's not the majority at all. 99% of the people use it to get rid of double cars and will simply list for whatever default the game suggests.
Oh, and did I mention the gifting system in FH5? You can just give cars away to random people, one of the most fun features in the game.