Typically I'll offer what "I BELIEVE" is a fair price and that it irregardless of whatever they have it listed for. But when I have a fair market price in mind I also take into consideration the fees and shipping and who's paying them. Most people aren't insulted but will come back with a counter-offer of their lowest price. Then you either barter or thank them and move on.
. I'm not a charity and if they can get it elsewhere for less, buy it. I'm not rich and I'm not a retailer.