Buy classical in multi-volume boxed sets. Many never got played even once.
Roughly half of our 3,000 LP's are these, nearly all bought off ebay. At least two thirds were never played before we received them.
Supply is often good, demand often thin, so prices can be low.
As always, it helps if your timing is good and you understand what you're looking at. People pay hundreds of dollars for Pierre Fournier's stunning performances of the Bach cello suites. I spotted them buried amongst an 11 volume, 120 LP Bach collection on pristine Archiv Production vinyl. Bought the whole collection, never played, for $.50 per LP.
Of course it helps to like music that lacks mass market appeal, and to have alot of shelf space!