I want to know this as well. Deal with this so often, and sometimes the window is short when you can get a good sale price.
Because people are either 1) jackasses or 2) kids using their parents accounts.
I remember one time I was selling an item in bulk and a quantity of 6 and a single user individually bought 4 of them. When I saw they had as 2 feedback and didn't pay for any of them, I knew there was about a .00008% chance they would ever pay. Usually buyers just ignore you when you ask if they will pay, but this user responded when I asked. They said "sorry my friend used my computer when I was in the bathroom and bid on a bunch of stuff as a prank. Sorry" in 15 year old grammar.
So I assume that goes on a lot in one way or another. Sometimes people bid on an item to "secure it" knowing nothing really happens if you don't pay, and then wait a day to see if they can find one cheaper. If they do, they buy and pay for the other one and leave you in the cold.
I don't even bother waiting the 4 days to file a non-paying bidder report, and then another 4 days to close it. I just relist after 24 hours, and if they end up paying, I tell them too bad you waited too long and refund the money. I just grew tired of the BS and people wasting my time.