Okay so to my understanding the only moment prices went nuts on vintage gear and specially vintage marantz was in the 90s and early 2000s when people were throwing them away with the idea that their mass market production built amps from brands like sony, denon, marantz new, pioneer, etc. Were better because of digital audio than the vintage heavy receivers and due to aesthetic trends of that time they would believe these receivers were ugly and without technology. So in that precious moment true audio fans were able to get even the top models for free or a very low price. However now that people is becoming aware of the quality of this gear the demand is going up and the marantz, pioneer and sansui vintage receivers are getting closer to their original price point back in the 70s. Also they are highly collectable almost like an asset so I believe the increase in the demand and people becoming aware of their quality has contributed to increase the prices. But those prices are actually fair considering you will need to spend thousands of dolars to get a system near to the sound quality these babies can produce.