Aftermarket parts never really add value and a highly modded vehicle will usually be worth less than a stock one (at least with cars). This is why it's never a good idea to put $$ into a car you don't plan on keeping a long time. The right private buyer might pay more for certain high-end mods, but it's not something to count on.
Motorcycle example: I have a '09 Ducati 848 with around 11k miles. It has Termignoni exhaust ($2000), Ohlins TTX36 monoshock ($1200), Ohlins 25mm fork cartridges ($900), Ohlins steering damper ($450), Dan Kyle linear rear link ($600), Woodcraft rearsets ($350), carbon fiber fenders and key guard parts ($400), CRG mirrors and levers ($250), Rizoma Lux grips ($150), Comp Werkes fender eliminator kit ($120), solo seat cowl ($150), Speedymoto fork and frame sliders ($200), Oberon 29mm clutch slave ($150), Zero Gravity wind screen ($100), and other random crap I can't even remember... That's easily $7k worth of primo goodies, but there's no way in hell I could ever sell my bike for KBB + $7k. In fact I'd probably be lucky to get $8k at this point.
Moral of the story: Don't pour money into anything you ever plan to sell.