I think you need to find some new friends...
Or ease up a little. I had a sort-of friend when I was younger, we hung out a bit but I thought he was a tool, naturally because he was, and I don't think he liked me much either. We went shooting together a couple of times, each with our own pistol, and weirdly, after checking out my HK, he meticulously wiped it down before handing it back to me.
I'm glad he got his filthy oils off my gun for me, but after that I never talked to him again. It showed a very ignorant view of gun ownership in general and me as a person specifically. You might be playing my former friend's role by trying to run numbers for a gun before buying them.
As a non-FFL you don't have access to the NCIC system, for excellent reasons, and when buying private property your options are these- Know the entire history of the item, like if a close friend bought it new and you were one of the first people he told about it, or exercise your judgement. Is this friend of your friend known to associate heavily with gang members or burglars? If so, maybe it's not such a bad idea to pass on the purchase, even though your personal risks are extremely low.
Or is he just someone you don't know as well as you know your friend? If so, suck it up and put the thought of running into trouble over the guns out of your mind, because a situation that would result in trouble for you beyond losing them is far-fetched, and the chances that they are stolen and that you'll lose them in the future is pretty miniscule as well.
I'd say unless you have some very specific problem with the current owner of the firearms, don't worry about them, lots of people sell their guns every year, many of us don't like the super-lowballing we get from a store, so put 'em up as for sale by owner. The other guy is probably just happy to be selling them to someone he knows at least a little personally.