When I purchase any used audio gear, I value anything at least half of MSRP. Just my thought.
Since FM tuners just aren't popular anymore, just a few hold value-Marantz 10B, REL Precent, Sequerra maybe the top level MD's along with a couple others.
I have a tubed Macintosh MR71 that was $400 in 1965. They can be found for $7-800 in decent working condition. Even mint I wouldn't pay more than $1K for another-and I'm quite fond of my MR 71.