Don't be afraid to ask questions. But most of all, don't feel that you "must" do the deal. Use your discretion. I met a guy in a Home Depot parking lot to sell a rifle, but if he asked me to meet him at say, some back alley crack shack I would have said no.
If you do have suspicions, and you're meeting in person, arrange for it to be in a public place, ideally one that has cameras. I remember back in Calgary some kid asked me what time it was, if he could borrow my phone for a second, if I had a smoke, if I had a lighter, for a few bucks, and he was obviously just waiting for me to pull out my wallet, or my watch, or my phone. I didn't have an at the time, so I told him I'd give him $20 if he came to the bank a block away with me. He ran off.
Also, one thing you can do is click and drag the picture into the google searchbar to search for other copies, or similar pictures. That's not necessarily an indicator of foul play, but it can give you a head's up if you see some guy in Quebec selling a rifle that's in a picture taken at a Vancouver range. That happened not too long ago.