Also, I know of genuine people (not being sexist here) but they are girls who have bought cars because they're nice looking, and don't even realise they are modified. Unless the seller tells them or a friend like I did to them, they wouldn't have a clue! :/ I think it's about time that people were insured, not the vehicles themselves (like in the US).
Also, dealers sometimes do their own options, I work for Nissan in a Plymouth and know for a fact they have a special deal with wolfrace wheels and fit these to cars where as 99.9% of new owners wouldn't have declared these... The buyer as far as they are concerned just presume it's a standard upgrade if they aren't clued up.