I know this is old but I've rapidly gone thru a few vehicles and systems recently. I voted yes but to me it depends.
Dealership: If I leave anything in there it would only be because of mods that can't be reversed and I'll put crap gear in there that fits. (Was in a time crunch and bad place because of a very sick Grandmother and a divorce and a move back home so I traded the truck with stereo and lift on it. Regretted it later because it was good quality stuff but only after my emotions settled and logic re entered.)
Private Party: Sure, for a FAIR price and if they aren't into this hobby then again, replace good stuff with less expensive gear.
Friend or Forum Member: Sure, for fair price, trades, etc. lol.
Like Chad said, it was hard to swallow the fact that as tasteful as mods to a vehicle can be, only 1% of people will be interested in them and only 1% of that 1% will be willing to pay for them.
Ive never competed so I'm not sure about those ramifications but my ego would prolly be butt hurt if I did and someone beat me with My set up. Lol. Only for a min tho.