You do realize that the owners of the things they sell are the ones who commission then to sell their things for them, right?
Anyway, the problem isn't auction houses, it's the ultra-wealthy driving prices of the things they desire to insane levels. And the reason it's a problem is because those prices are totally artificial and grossly inflated, solely for said ultra-wealthy to hide their wealth in the artificially expensive things they buy and avoid paying taxes that would benefit you and me and the whole of society.
Imagine having enough money to be able to waste it on a dinosaur skeleton for no reason other than to fucking have it. And it doesn't even make a difference in your net worth. This kind of shit makes me so damn depressed. I mean more than I already am.