This guy was a friend, but it still sort of bothered me because it might mean other customers believed it. He worked at a shop, and firmly believed that, to cut a shotgun down to under 18", all it would require is a $5 tax stamp, and that you could apply for and get the stamp AFTER doing so.
I had another guy try to sell me a Nagant revolver for $170, when they were $69 from J&G, and told me it was "well worth it," and he could get more.
I bought a broken shotgun from a pawn shop after being told that it worked perfectly and I could bring it back if there were any issues. I brought it back and they refused to do anything, even to give a partial refund. It wouldn't even load shells, but I wouldn't have known that unless I brought snap caps into the shop, and that was my first 12ga purchase so I wouldn't have had a reason to own them.
I had a shop guy tell me that my Remington 1900 is worth less than $150 in any condition, because they're mainly "owned by farmers who don't have anything else."
I check out a lot of shops and consider buying and selling often (not as easy as with other hobbies, such as buying and selling music gear, which I do constantly), so I'm sure there are more, but those come to mind.
I think the issue is two-fold. I've noticed that with the firearm interest comes a lot of fairly outdated practices and mindsets. A lot of the guys that are still around are of the older generation, and are often very set in their ways, irritable, treat younger people as lessers, and approach selling used firearms like used cars. It's worked for them before, so they continue to do so. I've probably met more crotchity, angry gun shop owners than friendly ones, and I'd say I'm a pretty friendly customer.
The second issue is that I'm younger, I am small, slim, and I have a fairly passive, entirely intimidating tone with strangers. With this older generation of people who think they can fool the lesser experienced, I think they try their hardest to pull beginner's tricks on me. A year or two ago, it might have worked better, but buying and selling a lot, and thanks for forums like this, I think I'm decently informed on the topics I have interest in (revolvers and shotguns, I know very little about autos and rifles), and I've had a number of people lie to me to my face, whether it's from their intention to trick me into a sale or just out of their own ignorance. But you also can't tell those people they're wrong, especially when you're younger and built like a kid.