Diablo – way to compare your currently equipped items with those in the auction house

As long as the item does not need to be identified, you can hold Ctrl and hover over the item to show its stats. If your inventory is open as well, it will compare the ground item's stats with what you have currently equipped.

If you hold Alt and hover over with your inventory open, it will compare it to your off-hand if it is a weapon that can be dual-wield.

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%.