Diablo – Do auctions time remaining still count down when the auction house is offline

As far as I can tell, there isn't one. I purchased an item at the auction house using Buyout so that I would know immediately when the auction ended. After a few seconds pause my buyout was accepted and the item was available in my Completed tab. I clicked to move it to my stash, and then went to the Sell tab and was able to immediately put the item back up for auction.

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%.