I bought a decent Marantz 2285B years ago for under $400 on Ebay, probably 20 years ago, and the asking for these nowadays is getting close to $2K.
Similarly, I purchased a basket case 2235B for around $200K, put a bit of sweat equity into it and now it’s worth about 3 times what I paid for it.
I purchased a classic 1060 from someone who didn’t want it for $200, I need to clean it up a bit, and I’ll probably just replace the main filter caps, I wont’ sell this one since it’s such a sweet little Amp with the classic Marantz sound.
I have a Yamaha IR on my workbench right now that I need to do something about, but unfortunately, the front panel has some wear on it that will make this more a labor of love than something that will increase in value.
Marantz was the Cadillac of receivers in the 70’s for good reason. They also looked great, something that is severely missed in most of today’s gear.
Bottom line is that certain brands from the 60’s and 70’s have continued to go up in price because of reputation and looks, and others may be better technically, and probably overpriced for what you get, but they’re going up at roughly the same rate as the stock market.