If you can, then keep the original parts.
Then when you come to sell you can return the car to stock, sell that at normal money and sell the after market parts to get some of the money back.
If you sell them on the car then you will not get as much back (if hardly any)
and unless the car is heavily modified to a particular taste (track day spec, rally car, drag car etc) then it is usually harder to sell on than a stock car as it can give the (often wrong) impression that the car has been driven and used heavily.