I've asked before - because as long as you're 
polite about it, there's no harm in asking. Or at 
least thats how I feel. :) There was a cel I was 
interested in, but it was 12,000 yen... the 
seller explained to me that they had purchased it 
at a high price and so couldn't really do much 
better, but they did offer to knock 2,000 yen 
off. Unfortunately it was still too expensive for 
me. 

So I'd say its at least worth a polite 
inquiry. :) You never know, the seller might feel 
its better to keep the stock moving and give a 
little discount then be stuck with something that 
isn't selling.