The best of Goodwill...

I know someone who found a clean Marantz 8B at a garage sale for $75

have not been in a Goodwill store myself...

Found some really good LP's at good will stores.

I have purchased for a friend:
Marantz 2270 in great shape, still in use after at least ten years. ($25)
JBL 'classic' speakers. The woofer surrounds were damaged so I made a new surround out of duct tape, same person has them as Marantz, also still in use. ($30 pair)
Ditto NAD CD player ($5)
A couple of Dual TTs. other speakers that seemed nice I sold for what I paid to other friends.
I missed by two minutes a Denon Primary 100 disc player, the really good one. ($20)
These were from the good old days.. before rummage sales pop up on every other block with craap they bought at Goodwill and are trying to resell. And before eBay where the same thing is happening, and before 15% of the population lost thier jobs and no one is donating much worth anything anymore.
I used to also buy Lps there, but I no longer bother checking out Goodwill type stores.

A first pressing of Sticky Fingers in scratch-tick-pop-free condition, with all artwork and inserts for $0.99. 2 mono-pressings of Wilson Pickett, which have to be heard to be believed. Again, $0.99 each. Mono-pressings of early Bob Dylan. Many other lps, too. Never found any decent electronics, but for me, Goodwill has been a great place. What you find depends on where you live. I live in an area of Florida where a lot of Baby Boomers are divesting themselves of their past.