Occasionally when I have seen some good ones on Ebay I have bought them to use later, but that was back when I built a lot more guitars from parts. I especially used to grab Fender Lace Sensors and used Rio Grande pickups when I saw them for a good price.
Not bought, but given. Back around 1972 a friend of mine had a 1956 Gold Top that he was determined to convert to a Deluxe (mini buckers). I was there when he removed the original P90's and he offered them, with the pots and selector switch, to me. I put them in a cigar box and put them in my parent's attic for future use. 2011 rolls around and I picked up one of the Les Paul P90 loaded Traditionals. A little later I remembered those old pickups, now stored for 40+ years. Getting everything together, I took the Trad and the P90's to my local repair guy and had the 56 P90's, and all the original wiring installed. It might have taken 40 years, but I eneded up with a treasure.
As a scratch builder, all the time. I'll almost always have every component needed before I start building. It takes me a couple or so months to finish a build from start to finish, so the hard part is waiting to try those pickups.
I have a couple sets of McNelly pickups that I'm still at least 3 months away from trying
I have probably five whole guitars worth of parts i could assemble if i needed something different including pickups for maybe ten or more guitars.
Its like kitchen supplies, i dont wait until Im making a sandwitch to buy mustard.
No, but I bought a neck with no guitar just because I like Cabronita necks a lot and one of the TDPRI folks was selling one. After a few years I swapped the neck into a Squier Bullet Tele, really worked out nice, then swapped the Squier neck into an old Chinese T-body. I think it's the "circle of life."