Wow, I have many super rare pieces in my collection. As for pieces that are so rare they never appear on Ebay Your talking holy grail games like Stadium Events or NWC90. I have many games to Extremely rare consoles like Gakken TV Boy or Casio PV-1000 that rarely ever appear on Ebay. I also have several homebrews for many systems that rarely appear on Ebay. The rarest games I own are the two Atari 5200 Prototype games Looney Tunes Hotel (plays exactly like Mario Hotel on CDi) and Sports Goofy.
But I also have super rare games like Dynastic Hero on T16CD, Keio Flying Squadron on SCD, Snatcher on SCD, Hagane on SNES, Final Fight Guy on SNES, Clay Fighter Sculptor's Cut on N64, Shantae on GBC, Musha on GEN, Panzer Dragoon Saga on SAT, American Mensa Academy on Wii, LSD on PS1JP, Pepsiman on PS1JP, Strip Fighter on PCE, and Racermate Challenge II on NES Just to name a few. I'd call these super rare but not holy grail games.