I once sold a bike to a chap on ebay who was fine until it came to payment time. He argued about paying, tried to drop the price, claimed he'd tried but it was blocked, tried to make out I was messing him around. He got really aggressive, swearing and name calling in capitals with lots of exclamation marks.
In the end I said fine, we'll meet in the middle, I'll bring the bike and you bring the money. I fully expected us to get in a fight and left a video camera running on the dash of my car when I parked up to record him smashing my head in with a pickaxe handle.
He was incredibly meek and mild, very quiet. Barely said a word, handed the money over quietly, muttered thanks and left.