Your mad/taking the piss/both I hear you say........
But if everybody took that attitude then thats how much it would be worth, you have to point the finger in a compassionate way, at the poor bugger who was first forced to sell his pride and joy for £15k, and poke your finger in the eye of the chancer who offered it.
BTW... my car deffinately is NOT for sale... even for £40k.... its too good.