Biggest lies I've heard are from people running down what I am trying to trade in or sell.
Outright fabrications.
"Buy low and sell high" is a sensible maxim if practiced sensibly. Otherwise, it marks you as a rapacious boob, and word gets around.
"It's not a factory recoil pad. That'll take off of the value." (It was a factory option at the time and very popular. The gun had the pad NIB. I rest my case.)
"That's not a Twelvette. If it was, it'd be worth something." (The early ones just say "Browning.")
"It's just an old double barrel. I'll give you two hundred for it." (Pietro Beretta was rolling in his grave.)
I understand horse trading, but when done to excess it becomes crass and actually harmful to your business. How is it harmful? It goes around, and then it comes around. People--who knows why?--ask me my opinion about guns and gunnery. I tell them not to deal with the guy at the corner of Walk and Don't Walk, or the other guy at Pedestrian Crossing. Drive out of town and deal with someone who wants you back as a repeat customer.
Then the fatal question emerges. The one that tests your own honor and integrity. "These things belonged to my late husband. I don't know anything about them. I always hated the things. What will you give me for them?"