When getting new parts, unless they're replacing a faulty component the old part goes into my second system, and the part replaced in the second system gets moved into "spare parts" box.
Generally the spare parts eventually get used (usually to fix friends/family computers), but should the box fill up I may do a clear out of anything i have lots of (often ram sticks/heatsinks) and either sell them or give them away depending how much they're worth.