First of all, it isn't his responsibility to find out if you can legally buy it. The responsibility is yours.
You can order a 11" AR15 upper right now from any number of places (as but one example). They don't care. They arn't doing anything illegal. If you buy one and can't legally own it, that's your problem. I have eleven tax stamps and have purchased a lot of stuff along these lines. ONE TIME I was asked to see the tax stamp before Hans Vang would sell me a 14" shotgun barrel. That is the one single time I ever have been asked if I could legally own the item I was purchasing. Again, that is my problem not theirs.
If a home invader was going to try to shoot me with a stolen Glock, I would hope they had one of these. The chances of them hitting me would be nil and in a second they would be out of ammo. Have you ever shot much full auto ? I have never fired a full auto handgun although two people I know own full auto S&W 9mm handguns. A good friend of mine fired one of them. I asked him if he thought it was possible to keep two shots on an IDPA silhouette target at 5 yards ? He said no. This guy that said no is an experienced shooter. He owns three machine guns and competes in our local machine gun matches. He has fired many, many different machine guns. If you doesn't think he can put two shots (on full auto) on a silhouette at 5 yards, imagine a guy with no shooting experience shooting a stolen gun under stress in the dark ?