If it was my bike and I could verify it, I would already have a police report on it, and the dude would get the bike confiscated and it would be back in my hands without having to pay the guy "for what he bought it for"... If you think your buying a stolen bike, just like a car...do the background research. If your bike gets stolen, make sure to file a police report and if you are interested in a bike that looks like it might be stolen, check the numbers and see if it is.
The guy could have bought it for $50, and now wants "$300, because thats what he payed for it"... Either way, it is nice that he posted an ad in consideration to his thoughts about buying a stolen bike, but his request for money and his demeanor about it is wrong.