I do that stuff sometimes. Most of the time, I reply with a lowball offer and genuinely explain why I'm giving a **** offer. "Look, this is what I'm willing to pay for this, and I know you grabbed this thing from the server room at work and it's free money, so just take it."
But today I saw a guy selling a "rare" 1984 Model M, and I showed him it was made in 89 and it was literally the single most common IBM keyboard ever made (1401). I wonder what he's going to do with that information.