I wonder if some of the lure of those 70's units is for nostalgia -- people trying to relive some of their youth. Pioneer, Kenwood and Sansui were often brought or sent home by Vietnam veterans.

There are also professional price inflators on eBay. They'll fake a sale at a very high price, then sell their 'collectors' inventory against that fake sale as a baseline for their discount.

I have a customer with a large B&O Master Link system. At one point MCL was the friendliest whole house system, but that was a long time ago. We did not install that system so I don't feel much obligation to support it, but I do need to cross swords with it occasionally -- especially since the local B&O store doesn't want to or can't support the system. Fortunately, there are some Links and controllers in our basement -- hopefully they still work if I need them. I'm sure that this customer was active in the design of the system (he's a smarter than everyone sort of guy) and does not want to give up on the system. Plus, he'd have some style issues with abandoned MCL's scattered everywhere in the walls.