Diablo – Same item, different DPS in game and Auction House

It isn't really an either/or thing, because there is no cost to a failed listing on the auction house. If the auction ends without any bids, you get the item back and can sell it to a vendor. So the only limitation is that you can only have 10 active auctions at a time, and you should consider using the auction house right away. You will make a profit anytime that you can auction for more than vendor price +15%, and as long as you never set your price lower than that, you can't lose money by trying to auction something.

My guess is that there won't be much of a market for magic (blue) items on the auction house, since rare (yellow) items are significantly better and will remain pretty cheap since they can be resold when you upgrade (unlike in WoW). So in practice, attempting to sell blue items will probably not get you anything, and you'll just end up vendoring it anyways, but unless you hit the 10 auction cap or needed the money right away, you didn't lose anything by trying. And there might be some blue items which can auction for slightly above vendor price + 15%.

Although I can't find official confirmation, I believe this error indicates that you attempted to buy something that no longer exists. This generally means that someone else purchased the item in question while you were browsing.

You'll get this error often if you try to buy popular/in demand commodity items (gems, crafting materials, etc), especially when the prices are far below market. The quantity of people shopping means that there are lots of buyers watching item prices and snapping up good deals.

Although the game will temporarily take your money when this error occurs, I've found that shortly afterwards it shows up on the "Completed" tab as a "Failed" purchase. In some cases I was able to speed up the refund process by entering and then leaving a game session.