If they do blind bidding like in XI, I would think they would have to get rid of custom stack sizes and only allow items to be sold in the quantity of 1 at a time, or 99. Otherwise a situation like dspguy asked about would come up. This way you would always know how many you are getting, and it's also a lot easier on the math.

Sadly, this means that someone who is buying only 20 or so of an item instead of a full stack of 99 would spent a lot more time bidding on 20 individual items. A plus side to this is they can try to minimum bid as many as possible until they run out of items listed below market rates. The biggest downside is that it would take a lot longer to buy up 20 items. Imagine if you wanted a half stack? 50 confirmation dialog boxes = NO THANKS.