what are your top "audio bargins" in your system history?

In 1982 I saw a pair of ESS TranStatics listed in The Recycler, priced at $400. They originally retailed for $1200 in the early-70's, when I had lusted for a pair.

The speaker is a 3-way, the highs reproduced by three RTR ESL tweeters (the same tweeters Infinity was using at the time in their Servo-Static and 2000A models), the mids by a KEF 5" B110 driver, the bass by the renown KEF B139 oval woofer (used by David Wilson in his original 1970's WAMM) in a transmissionline enclosure.

Still have 'em!

The early Classe integrateds deserve a mention.  CAP-100 etc.

Right now I can buy a Nakamichi CR-7A cassette deck for 800 dollars. I believe it's in good working order. Should I buy it and flip it?

To this day I have no idea why the seller priced a 9.5 out of 10 Rotel RHA-10 preamp at $300, but I do know it wasn’t stolen.

I hooked it up, turned it up, then looked it up; there was only one on sale in all the world, in Germany for $1300.

My whole system from the used market. Intergrated amp at 45% retail,  steamer-dac at 40% retail, speakers at 40%. 33 grand system for 15 grand. Plus 10 grand of cabling for 3. Only way to go on a disability pension over time. I was lucky enough to sort out getting my dream speakers when they became available, which was the game changer for me. Was seriously hard to do, but well worth it to me.