Best thing to do is ask for the serial number. If they are willing to give you the serial number, then you are good. On another note, you can find out about laptops that are stolen this way..HP, Compaq, and IBM laptops are still warrantied even if they are stolen and under warranty. Dell's as far as I know are the only laptops that can give you problem if you bought one that is stolen and you tried to call it in for warranty repair.
But just find someone that has a lot of feedback and read there feedback and what items they sold and what the feedback reply was.
I'll be honest with you, if the price is to good to be true, then either its an Out of Box sale, refurb, or stolen. Just ask for the serial number. If the seller is willing to give it without question then its legit.
I give me serials out in an instant. Gives the buyer a lot more confidence on buying your item.