As a seller, I would never put anything on the auction house because no one will actively buy something for higher than the lowest offer of that item on the market. One exception would be if someone places a high bid on your item, and then that same item pops up for cheaper on the market later. They can't retract their bid and no one else would overbid them so they're stuck.
As a buyer, I love the auction house because you might be able to get a bargain. Of course you'll have to keep this silly 5% tax in mind if you wish to resell it...