I think it depends on the asking price in the first place, and the type of sale.
In a face to face "Cash" sales environment I think offering 10-20% less than the asking price is fair (especially as this has probably been factored in when the seller priced it) however if its already cheap, and priced fairly (a few quid) then why bother!
On eBay I think its fair to offer the going rate. I try to price my models/items fairly (and often are at the lower end of eBay prices as I want a quick sale) so it really annoys me when people send me a message along the lines of "Give you £20.00 for it, its not worth much more than that" on a model priced at £50.00 and regularly sells for £60+ - These are chancers trying to make a quick buck and they must get some success as otherwise they wouldn't waste their time!