[yorkie_chris]

There are loads of idiots on drugs selling bikes.
There are even more idiots on drugs trying to buy them.

Shrug your shoulders and go have a pint or something. There are loads of idiots about and getting worked up over it will make no difference whatsoever. Treat it as a valuable learning experience.

Or maybe point out that they should expect to take a loss on stuff when they devalue it by crashing it, abusing it and generally trying to sell a heap. They'll probably tell you to p*ss off, and in that case I suggest the earlier advice of shrugging and going for a pint.