I don't know on what scale of "rarity"
Hot Blooded Family (a 1st generation Beat 'Em Up PS1 game) is, but I rarely see it up on Ebay. I mean it's up from time to time but not very often. It might not be so uncommon on Yahoo!Japan or other Japanese auction sites, but using Ebay as my primary procuring process for the globe (outside of gaming forums communities anyhow) I'd say that one is pretty rare. Though I know ole' Carlson (16-Bits) from Mad-Gear LLC has had a few in from time to time. The other game that I find is nuts to find is the Sega Saturn game
Psychic Warlock Assassin Taromaru. I had that one brand new imported from Die Hard Games no less back in '96 or '97 and sold it... still can't find one very easily these days and it's twice the price of what it was back in the day.