stolen guns

Question
i want to buy an ar-15 from a guy i know , can i run the serial no's. on line

Answer
No, not to my knowledge.

The best you could hope to do would be to contact your local reputable gun shop and ask them what guns have recently been reported stolen (the investigating agency will obtain the makes, models, and serial numbers from the rightful owner IF he's had the forethought to record them, and send out a list to local gun shops in case they show up at one of them). But these of necessity will be local (e.g. a gun store in SC would not about guns stolen in NM).

Most people, when they engage in private commerce of guns, have some knowledge of the people they're dealing with (it's a relative, a friend, or a relative or good friend of one of your friends). If the seller seems shady to you, or you suspect the gun is stolen, I would pass on it.

ADDENDUM: I just checked with a gunshop owner I know. He also pointed out that there is nothing proprietary about firearms serial numbers, and it is entirely possible for two or more guns to have the same serial number (in which case, things like make, model, manufacture date, and other distinguishing features become the means of identification). So I restate my advice - if you have questions about the integrity of the sale or seller, I would pass.