I usually don't bargain, but I have made a few 'unreasonable' offers before now. In one case I couldn't afford the asking price anyway, and I thought maybe the seller would prefer to sell it to me (it was a piece of furniture with some sentimental value) rather than send it to a brocki-haus. Didn't work out (someone else gave him a higher offer) but he wasn't offended by it either.

In another case someone was leaving the country next day. So she could sell the item to me for CHF20, or throw it away for CHF0. When you look at it that way my offer was a win-win and we both knew it. She'd originally asked 60CHF - slightly below retail, yes, but not enough to be worth the hassle on such short notice - so in effect the discount was the 'reward' I claimed for being flexible.

I think you can be pretty cheeky with offers as long as you're unfailingly polite about it. Doesn't hurt to admit up front that you know you're being cheeky but here is your offer. Then they can take it or leave it, without feeling outraged.