@Niwashi
The problem is, the 'whole stack' no longer exists the moment someone buys the first item from it.. so it would have to be an option to 'lock' the whole stack into single unit. But that doesn't make it sell any faster, because it limits the buyer's options. Buyer doesn't care about your space, only about price. If you want to clear out, you just put up lower price than regular market value. Doesn't matter if one person buys it all, or if 10 people buy it by pieces.

What I'd really want to see is 'buy orders' included on the list - that is, offers to buy things, as well as offers to sell things. That would clear your problem as well - since you could simply sell the stuff you want to clear, to people who have listed an offer to buy them. The prices in a system like that (one that has both buy and sell offers) will stabilize over time, and the difference usually ends up to close to sales tax (if the difference is significantly higher, someone will put up a higher buy offer, and then put the stuff for sale as soon as they get it).