What components are a good value in used market?

Are you referring to speakers only in the 70's?
I'm not sure what your asking?
Dalquist DQ-10
Quad ELS Speakers
Acoustat Model 2+2,s
Pyramid Met 7's
Vandersteen Model 2's
Spica 50's

I have seem my amp the Sonic Frontiers SFS 80 go for about 1K, retail in its day, 2795. Had mine for about 12 yrars, only problem is that I had to change the fuse, along with the tubes. Excellent Tubey sound, tight bass, good imaging when properly matched

BERNING

goldmund dialogues if you can find them

The best value is Phono Cartridges, for goor reason, they are very high risk. You have to trust people and their descriptions. I bought a Koetsu Rosewood Signature virtually new for $1500, but you have to be brave.
Secondly, cables of all sorts are cheap 2nd hand. Useful as the only way to test them is over time in your own system. Sell them on if you don't like them. I just bought an Audience power chord less than half price, not only great sound, but sorted a transformer hum in my integrated amp. Why it did, I don't know, just a nice bonus.