Not sure if I would bother either.
Presumably they have seen the car and if they did not ask specifically if it had been modded then should be no problem.
I recently bought my 350Z from a reputable dealer. It has some mods and they never brought them to my attention.
I also had to sit through the usual crap of 'we are phoning around for the best price on your p/x', which was actually a very nice (but old) Z4, and then they just sent it to auction anyway.
The Z4 had loads of extras including private plate, hardtop, couple of different wind deflectors, indoor/outdoor covers etc all that were thrown in when I bought the car. Garage made no allowance for these in p/x price so I removed and sold separately. Hardtop £1K, plate £1.2K, other bits £600. I did advise garage that I was doing this, they did not care less, in fact got a 'well done' from the salesman.