The answer is patience. In general, I don’t scour the used market for areas of need, but for opportunities to acquire interesting stuff at a bargain price.
Sure, you can focus on amps when you need one, etc., but in general, I have a slush fund budget for used gear when a good opportunity arises. Craigslist is great for that if you are in a big enough metro area. In the Chicago area, in a decade I’ve built, traded, sold, and rebuilt systems for less than half retail/new.
True, sometimes you get mistreated gear, Investigate first, and don’t buy or bargain downwards if so. A guy on Craigslist had a Nakamichi PA-7 for something like $450 and I negotiated it down to something like $325 but a prior owner had drilled big circular holes in the top cover. Still sounds great. Just got a pair of KEF R11s for $1850. Still available new for $6000. Live with the warts, enjoy the sound!