I think that would be best, yea, i just will miss MB.
A) I would mention it but write something like 'as with most used mountain buggies there is a small amount of rust on the frame, this does not affect the buggies use and is barely noticable, I just like to be honest'
b) depends on how bad it is, if its just little patches I wouldn't bother.
c & d) no sell them to me! lol
£20 seems fair I put £16.99 for mine.
If you do sell you bumper seperatly and its a single one I would be very interested, and also in the liner if you have pics please