I wouldn't give anything back, the guy sounds like he's looking for a perfect 20 year old bumper for a cheap price, your not gonna find it, so just take the loss and buy another bumper, you'll probably find out the first one was in better shape, your not buying new parts, defects big or small is a the buyer's risk, and the buyer should be aware of the risk, maybe you should of looked at the part closer to you so you can go assess how much it was worth, no picture could display every defect, the seller should not pay, go to any salvage yard, they will not replace something like that