I am looking to purchase a Smith and Wesson 637 from someone who lives in the same town as I do...
I think it is safe to assume they live in the same state.
No, unfortunately, its not safe to assume anything. There have been a few threads recently about legal residency, and this kind of thing has been discussed.
Sure, if he lives in the same town you do, he probably is a legal resident, but you shouldn't just assume he is. The seller could be just short of legal residence status (unlikely, but possible) and if so, then a face to face sale would be a violation of the law. Seeing a driver's license verifying his residence is never a bad idea. And if they won't show you one, you have to wonder..why?