If someone make an auction they can't put that item in market.Why they're not allowed to just put autobuy on the auction thread and put that item to market with autobuy price?
sorry for my english

Because bids are binding. If I bid 20k on your item, and you sell it on the market, that's unfair. Because had you not sold it on the market, I could've won your item for significantly less. And had I decided to not buy your item, I'd owe you a small % of my bid.

You as a an auctioneer can't force people to have binding bids if you can't assure them that you'll even have the item to sell.