I keep it all, never know when I'll get the itch for a different car and want to sell it. Always found it easier to sell the car when it's back to stock than with parts on it.
Sold most of what i had and put the rest in storage... Old brakes, stock swaybars panhard bars,stock mufflers i still have. Even my stock hood is in storage! Maybe we should pool our parts and start a back to stock store?..lol
I've kept everything old that I could not sell, though I'm getting tired of storing everything and just about to throw it all away. If I ever do sell the car, I'd probably swap the parts that are worth re-selling otherwise they're staying on the car.