It depends on how you are planning to sell.
I took my Milltek Cat-back, Neuspeed intake, Forge DV off as it made no difference to the PX price. I even kept the Pirelli logo'd mats that my wife got me. Already sold the exhaust. Left the WALK on as it wasn't worth the hassle.
If you have time and patience, then sell it complete - you will get more for it (from the right person - eventually). I didn't want test pilots to be wasting my time because they just wanted a shot of a Revo Stage1 Pirelli.