trouble is, they are hoping for someone to be too tight to bother hpi'ing and they are out there.

So, by saying I don't know, they are not lying, just not being truthful and they hope you'll just buy without checking, and if you do, they haven't lost anything from not telling you.

Sellers in the main keep the bad bits to themself, and the worst bit is, if you DO tell someone the bad bits, they don't turn up for two reasons:

a) they think that there will be more than your telling them
b) another seller will be lying and claiming MINT, and so they think the other car is better than yours.

Just can't win.