I don't collect, but I do have a stockpile of spares: early high end Suntour, later Suntour Sprint and Superbe Pro, Shimano 6400, 7400, and early Deore XT. All of the Shimanos are groups I tend to use on keepers.

I also save up anything touring oriented from the 1980s and 1970s: long cage Vx derailleurs or any of the other better Suntour long cage RDs, Suntour bar cons, early Sugino triples. As my stock of these items grows, I tend to sell off a few on that evil auction site to keep the inventory in check and to replenish the bike fund. Campy bits usually go directly to the evil auction site to help offset my bike spending.
Mid level stuff typically gets put on flips, low end stuff tends to go to the co-op.

Best source for almost all of these parts are donor bikes.