I've come across that a few times as well... people with machines that are stuffed, thinking they are worth a mint!
Thankfully, more often than not people still think the old machines are not worth much... this only helps when buying, not selling!
But you are right about the eBay thing Gavin... I suppose you could say that "the machine you saw for $20,000 never sold, as it is not worth anywhere near that much"... and tell them to go look for themselves.
Some people always believe what they see/read, so you don't have much chance if this is the case.