I have quite a few rare albums that fit your description.
Most of them are 1970's Italian prog-rock.
Because of their love of classical music, music training in public schools, and the high level of musicianship, Italy really took to prog-rock. There were many bands that produced one incredible album, then for whatever reason, were done. Italy had many bands that were just as good as the better known British bands (YES, King Crimson, Genesis, Gentle Giant, ELP, etc) but due to non English vocals, they did not receive the worldwide recognition they deserved. When Genesis was barely known in the States, they were selling out large venues in Italy. Even bigger than they were in the UK.
If you ever run across these, and you are lucky enough that the owner has no idea what he/she has, buy them!
De De Lind - Io Non So Da Dove Vengo E Non So Dove Mai Andrò, Uomo È Il Nome Che Mi Han Dato
This consistently sells for between $1000 and $2000 on the collector's market!

Murple - Io Sono Murple
Another killer album by a one-and-done band. These go for about $600 on the collector's market.

Il Baletto di Bronzo - YS
This one is a bit controversial among prog fans. Some consider it one of the best prog albums ever made, others, not so much. I fall in with the former group.
It's about as dark and menacing as any prog album of its time. Geoni Leoni, the keyboard player, is easily as good technically as Keith Emerson or Rick Wakeman.
These sell for about $500.
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Alphataurus - S/T
This one, for me, does not hold up as well musically as the others I've listed, but it is super rare. Regularly selling for about $1000.
It has a great triple gatefold album cover.

Semiramis - Dedicato A Frazz
And yet another killer one-and-done band.
Sells for $1000 +.

I could list many more from Italy. And many from Germany, France and Spain also.
Spain had a particularly interesting prog scene, since many bands beautifully integrated Flamenco into their version.