RARE in pinball terms means not many were made.
99% of the time this means it didn't do too well on site.
4000 games made isn't rare.. 350 made is and means the game is most likely crap to play

In a nutshell.

eBay "rare" - especially from a dealer - translates as "We don't don't know much about the game, so it is rare". Bally "BMX" is rare. Stern "Viper" is rare. "Star Trek" and "Space Invaders" are EASILY found. I think we have 5-6 Star Treks..So it is pure stupidity to suggest Bally "Star trek" is even remotely "rare".

I collect rare games ;

GTB - Spirit, Punk and Rocky. Rocky is rare, but is highly desirable - making it more rare to obtain.
Bally - Six Sticks (only English version I know of in the country), Embryon, Viking and Vector. Boomerang EM.
Stern - Freefall
Williams - Winner

"Rare" is literally low production run, but can be games that are highly sought after, and not easily obtained. Safecracker comes to mind.