Not necessarily. There may be other circumstances in which the serial number might be checked. And in many ways the practical concern is that if you are found with a stolen gun, even if it's absolutely clear that you came by it honestly and innocently, the gun will be taken from you to be returned to the rightful owner; and you will be out the money you paid for it.
This is specifically why it makes me nervous. The gun I was looking at buying was going to cost me $800 and if by some small chance it did have to be checked for some reason and came up stolen I don't want to lose that money which I would. Also, its not that I don't trust my coworker. My problem is I don't know the guy he bought it from or the guy that person bought it from and so on. I decided not to purchase the gun just in case. I think I'm going to stick with my buy new philosophy.