Girvin flexstems. They were dire, an answer to a question no one asked. Much derided and unloved then, now go for daft money.
Full squidge Marins. Since about 1995 Marin have been living off the name, and none of their full squidge bikes has ever been competitive in weight, ride and performance to this very day, but people are prepared to pay silly money for them.
Turbo saddles. A tenner in Hellfrauds in the lates 80s, 50or more nicker for a very smart example now. Good saddles, but that's ridiculous, especially when the charge bucket is an exact replica for under a tenner if you shop about (and also available in funky finishes if you like the classic cut but with contemporary colours).
The Retrobike parts market is a funny place that seems to exist outside the realms of the laws of physics. Stuff that was tat then is still tat.today but people fall over each other to pay big moolah while good stuff is overlooked. And don't get me started on the strange meaning that "NOS" seems to take on in this parallel universe...