i don't know you, and have never dealt with you, but i've read the entire thread, and disagree with your line of reasoning, just like everyone else who's posted in this thread. if he called you that day, told you he'd be by AROUND 3pm, and you sold it at 4pm, that means you sold it out from under him---plain and simple.
correct me if i'm wrong here:
you guys agreed on a price
he agreed to buy it
he called you that day
said he'd be by around 3pm
he showed up about an hour late
you had already sold the computer
did you warn him that you would sell it to the first person who showed up with money in hand?
if it happened to me, i'd start a thread to warn other people here, regardless of anything i said to you in person
just my opinion