I do exactly as you do. I have made quite a few mods to my cars over the years. Springs, exhaust, dampers, stabilizer bars, brakes, wheels, ect. If this is a newer car you may re-sell, keeping the parts and then putting them back on the car when you sell generally helps re-sale. Normal buyers don’t want your modifications, it scares them!
If you, like me, have no space for storing the old parts, there are very limited places to rid yourself of parts. Brake parts, springs and stabilizer bars and dampers go straight to scrap steel recycling. No one wants these parts, you won’t be able to sell them. I’ve tried. Intake parts and other soft interior parts go into the trash for the same reason. Aluminum parts are well worth taking to the scrappers for recycling.
Wheels and tires and seats sell well on Craigslist if they aren’t just horribly ugly… ECU’s and other electronics - Ebay is the only place to get a wide enough audience.