Yes, definitely contact the FFL for sure. If the guy is really going around flashing someone else's FFL then he's proly looking at some prison time. On the other hand, lots of people have FFLs even though they don't own a gun store. They just sell guns out of their houses, he could be doing that.
Be careful how you proceed with him. I've encountered some mentally ill people when selling guns in the past.
I had one situation very similar to what you are describing. I was selling a gun for my dad because he's not really gun savvy, so I help him out with that stuff. The guy made us a top dollar offer, and I told him I would hold it for him and meet up with him that weekend to make the deal. That wasn't fast enough for him, though, and he wouldn't take no for an answer, kept bugging me. I explained to him that it was my father's gun, and that he would not be available until that weekend because he was out of town on business. The guy wanted me to go get the gun out of my father's gun safe, then meet up with him that day, to which I explained that I wasn't about to sell my dad's gun without his knowledge, to make a very long story short. Well, the guy didn't like that answer and he started harassing me, told me I was a pussy because I wouldn't sell my dad's gun without him. Keep in mind that it was only like three days until my dad got back in town. In reality, my dad wouldn't have cared if I had sold it without him, and I do know the combo to his safe, but by that time I was getting spooked. The guy was acting totally schizo. I tried to be polite, just told him it wasn't going to happen until that weekend, hoping he wouldn't call me back and I wouldn't have to mess with him. He just wouldn't give up tho, so I told him to stop. He sent me threatening text messages for the next two days. I looked up the number and found out he was living in a halfway house for mental patients.
Definitely don't send him poop in the mail, that would be a federal crime, they could even hit you with bioterrorism or some bs like that