It's likely that the seller (and many others like that) have had their items appraised by some crazy outfit like Antiques Roadshow or Pawn Stars, who may give a value for the item what it "might fetch at auction on a good day." Unfortunately, the law of selective hearing dictates that "might fetch at auction on a good day" equals "is worth at least that much and not a penny less."

No one likes to think or be told that their old junk is worth less than what they would like it to be, and they probably honestly think that they'll get their asking price when they put it up on eBay or similar places.