felt like sharing a current list with this thread so here are some that come to mind, sorry if some of them were mentioned already:
--- usually available but expensive --- (these usually sell/list very expensive. sometimes i've seen some for considerably less than usual, in that case it's noted next to it)
(definition of expensive here = around or above 10000jpy)
A-Train 6 Original Sound Track
Akagawa Jirou Yasoukyoku -Original Sound Collection- (seen two times for around 10000jpy, apart from that only for much more)
Astronoka Original Soundtrack
BLADE MAKER ORIGINAL SOUND TRACK
Bounty Sword Ongakuhen (seen once for around 10000jpy, apart from that only seen for way above that)
Choro Q Marine Q-Boat Original Sound Collection
Cyber Doll Original Soundtracks (actually was on suruga-ya for around 7000jpy a short while ago)
Death Crimson 2 -Yuri Rosenberg-
Dokapon! Ikari no Tekken Original Sound Album
Dynamite Headdy (saw it for around 6000jpy once, either Yahoo Auctions or Amazon)
EVERGRACE Original Soundtrack (saw it for 5000jpy once on Yahoo Auctions, uncomplete i think)
Fushigi no Yume no Alice
Gakkou de Atta Kowai Hanashi S Original Sound Horror
Glory of Heracles ~ Greece Reminiscences
Glory of Heracles IV ~ Gift of the Gods (there was a copy at Yahoo Auctions that, if i remember right, sold for around 7000jpy)
Gungage Original Soundtrack (saw this for around 5000jpy once, i don't remember where)
Insector X (currently on Suruga-ya for ~8000jpy and was once on Yahoo for around 4000jpy not too long ago, but usually lists for more)
Island of the Seven Winds Story Original Soundtrack & Arrange Album
Kaeru no Ehon ~In Search of the Lost Memories~ Original Sound Track
Kokumeikan Original Soundtracks(mostly seen for way above the 10000jpy mark, but at times it popped up for around 10000jpy, never seen for less than that)
Let's Take the A-Train Z Original Sound Track (there was a copy once @ Yahoo Auctions around 5000jpy)
Maboroshi Tsukiyo Original Soundtrack
Madou Monogatari Original Sound Track
Majotachi no Nemuri Original Sound Track
Pilot ni Narou! Original Soundtrack
Sayonara no Hohoemi The Original Soundtrack
Shin Momotaro Densetsu(online only seen for way above the 10000jpy mark. according to a pal this has been available for 6300jpy in a store in Japan not too long ago)
Shutokou Battle Original Sound Track (actually seen two times for around 5000jpy)
Shutokou Battle ZERO Original Soundtrack
Silent Bomber Original Sound Track
Sound of Tower
Sword Maniac
Tantei Jinguji Saburo: Yume no Owari ni - 10th ANNIVERSARY SPECIAL ALBUM (saw this for around 5000jpy once on Amazon)
Waku Puyo Dungeon Ketteiban Original Soundtrack
WILDTRAX ~ RARE TRACKS FOR DRIVING (besides those crazy offerings it appeared here and there for around 10000jpy. there was a constant listing @ 5000jpy, which was a sample CD)
Wonder Project J ~ Mechanical Boy Pino ~ Sound Adventure
WONDER PROJECT J2 ~ Josette of Corlo Forest ORIGINAL SOUNDTRACK
Zeus II Carnage Heart Original Sound Track (never seen for less than 20000jpy)
--- very rare (only seen one or two times) and expensive ---
Chaos World
Iblard ~Laputa no Kaeru Machi~ Soundtrack
Rising Zan: The Samurai Gunman Original Soundtrack
--- very rare (only seen one or two times) but decent price ---
(definition of decent = not anywhere near 10000jpy)
All Sounds of Last Armageddon
First Queen Opening IN HOUSE (cheap and appeared two times recently)
MIRAGE SUPER ARRANGE VERSION (appeared two times recently for considerably less than that one 10000jpy listing @ Yahoo Auctions)
PC-Engine Original Super Schwarzschild 1 + 2 (seen for less than 5000jpy)
Popotan BGM Collection (cheap but insanely rare)
Suki na Mono wa Suki Dakara Shouganai!! FIRST LIMIT & TARGET + NIGHT Original Sound Collection "Ciel" & "Nuit"
--- very rare, never seen myself (but partially seen by others once)---
10101 "WILL" The Starship Sound Track
Chachamaru Game Music Human Omnibus
Cyber Daisenryaku: Shutsugeki! Harukatai Original Soundtrack
DEATH BRINGER
Death Crimson History
Fire Pro Wrestling G Original Soundtrack
GAOGAO! COMPLETE COLLECTION
Les Voyageurs du Temps
Motto my Angel Drama & Soundtrack CD
Only You ~The decadent juliets~ in memoriam Sound Collection
Pinocchia no Miru Yume Original Soundtrack
PMD98 (I *think* I've seen it on Suruga-ya or Yahoo Auctions once, but to be safe I put it here)
Satomi no Nazo Original Soundtrack
Sidewinder Original Soundtrack
Sound Collection Sold Out series
sound story of Eko Eko Azarak
The Miracle Garden Original Soundtrack
THOR ~Seirei Ou Kiden~
Wonder Beagles Original Soundtrack