sorry but the reality is that if the seller misled you you are well within your rights to ask for a discount because the alternative is that you return it back and get a full refund which the seller probably doesn't want. I don't care if it's a bike, a golf club, a car, or whatever. EBAY is a community and it's only going to work when both parties behave fairly. If 20 pics were truly posted with none showing that damage, I find that to be pretty intentional on the part of the seller who probably realizes the buyer may be a bit miffed but probably won't go ahead and complain. If this person sells a large quantity of items, it's worthwhile because maybe 1 out of 10 times someone will complain and he'll kick a little money back but otherwise still make a big profit on it.

Typically the feedback pertains to the smoothness of the actual transaction not the item condition so it's hard to determine what sellers are quality and what ones are shady