Is it good idea to buy a 20 years old Accuphase C-290V

Accuphase are second to none in their reputation for build quality and durability, as you note.  However, buying from a second-hand shop you typically have no information relating to how the piece has been used or maintained over twenty years.  $6K is a lot of money to spend and you might consider that the same outlay will get you a top-flight piece new with warranty, or late-model used.  It's usually between 20 and 30 years of age when components first require substantial attention, though this varies of course.  There's no denying that the C290V is a seductive piece.

If it makes you feel any better, I just bought a 20yo Classé CA-5100 which still cost me close to 2k.

Ultra HiFi is usually built well.

A good place to check older equipment reliability by owner reviews is audioreview.com.

Dear audiophiles,
Thank you very much for your kind reply. New accuphase high end models like c 2850 or 3850 cost much much more which most audio fans unable to afford. And some later high end models are located here and there which usage are unknown either.  If I purchase, I might have risks.  Though the one i found of C290 V is in the 2nd audio shop, but the shop provided buy back policy. You can return in 3 months and get 90% back or 1 year get 80% back.