Oh, well that sucks, I normally go ~15% should be acceptable for bartering... but then again each seller is a bit different, if he will only take a 5% discount, might as well price it there with no offer option and sell it quicker. But then again.. that's just my opinion..
edit: just a hair slow, and E29, sometimes if you dont need a board, but might be able to clean it up if the seller accepts it is nice to throw in an offer. And if the seller doesn't want to waste time on super low balls they can always have it set up to auto decline them.