The most often cited case involved a kid purchasing a Nolan Ryan rookie card from an unknowledgable clerk for twelve dollars instead of twelve hundred. Despite the fantasies, you can't purchase a cherry 1960's corvette out of barn from a naive widow for a few hundred dollars. For a transaction to be legit both parties have to operating from a somewhat equal position. That includes knowledge as to the value of the product involved. That especially holds true if the buyer holds themselves out as some kind of expert, like the pawn shop owner or the antique dealer. But like the mentioned case, it even holds true if you are a snot nosed little kid with a subscription to Beckett.
Taking advantage of the elderly does apply especially if they are swindled out of their item. But other than that, if a seller is willing to sell an item at a posted price, then there is nothing they can do later if they find out the purchaser got too good of a deal.
A person that works at one of my stops was swindled out of 35K--her life savings. She's not a very attractive woman who was desperate for male companionship. When her friend found out, she marched her right to the police station. The police said the guy broke no law. He asked her for money, and she willingly gave it to him.