Again, retail stores (or wholesale for that matter) do NOT have access to NCIC. That's the only listing worth 2 cents as far as determining if a gun is stolen. There are several databases out there promoted as good places to check numbers, but the fact is, they all require the owners to voluntarily list the numbers of any stolen guns. If the owner doesn't happen to list with that particular database, it doesn't show up.
The only worthwhile thing to do is ask the seller to go with you to the local PD and ask them to run the number. If they have time, they MIGHT do this for you. If they get a hit, the seller will lose the gun, but you won't be out your money. Personally, I don't think very many sellers would do this, but it's up to you.
I do recall a recent article on stolen guns stated that something like 10% of the firearms stolen had been recovered and returned within 5 years. Not very good odds.