I'd say get a receipt with the seller's contact information on it (verify it via ID or otherwise), and tell the seller that you plan to have the weapon's serial number run.
Usually the crimes surrounding stolen property can only be prosecuted if the prosecution can prove that the accused knew it was stolen and had it in his possession anyway.
If the seller doesn't back out and sells you the weapon, when you take it in to have it run, if it comes up as stolen, you can pass the seller's contact info onto the police.
If the seller won't give you his contact info and won't provide an ID for you to verify it, then that's a giant red flag right there. The other posters have a point though... if you have a bad feeling, no good gun deal is worth risking that you'll be caught with a stolen gun. If you do get convicted, you'll be banned for life from owning any guns in most states - not worth it IMO.