I've never been a fan of keeping used parts as spares. I figure if a part needs to be replaced, it's because it's a bad part and why would I want to turn around and put it back in my boat again? And if I did put a used part in the boat as a repair, wouldn't I want to replace it with new at the next opportunity, thereby having to performing the repair twice? No thanks.
I tend to treat the idea of spares as a "one size doesn't fit all" situation.
We replaced a water pump with a new one... but rebuilt the old one, and I carry it as a spare. A get-home that I could install underway if necessary.
With belts and such like, we usually do as FF suggests: on with the new, save the old. Another get-home, in case the newer one fails.
OTOH, with impellers, I buy new and install the spares... which were new when I bought 'em last year. There, I'm hoping to deal sensibly with the idea of shelf life.
We keep a full set of filters on board; mostly as a matter of convenience. If I want to change the oil on a Sunday, I can do it without having to find a chandlery with our filters in stock.
I don't have a spare starter, aftercooler, turbo, freshwater pump, etc.... Yet. If I need to replace one, and if the old one is rebuild-able (within reason), I might consider that, though. The freshwater pump we replaced was NOT worth any additional effort... but OTOH I can get a new one relatively quickly if necessary. (Coastal cruising.)
And so forth.
IOW, a firm "it depends."
Yes, sometimes that might mean a repair twice... but OTOH, I do most of the work so it's not a huge cost, other than the part itself. And sometimes, I just don't want to tie up too much $$ in inventory.
Still, I do think different circumstances -- coastal versus passagemaking, for instance -- ought to guide one's process for dealing with spares.
-Chris