I used to do a little trading on e-bay selling computer parts I bought in bulk. I used to give the best prices for them, and one time bent over backwards for one guy. He bought a GForce2 which was DOA for him. I then replaced it with a GForce3, with an upgrade price of about $75-100, taking all of my profit from some 50 cards I sold. I also had everything shipped next day guaranteed delivery, paying for both him to ship back to me, and me to ship to him, so he was without for about 3 days.
He posted lots of bad stuff about how he received a bad part, took forever to get his replacement, then, get this, had the damned nerve to [censored] that the replacement wasn't what he ordered !!! Damned cheek of the bugger, it was better !!!
Well, I don't bother with e-bay too much now, as that single comment destroyed my reputation.