I live near Ann Arbor, MI, home of U of M, and I think that contributes to the relatively large selection of good condition older equipment. A college sophomore sold me the EPIs -- his roommate had a pair of Dynaco A-50s, but wouldn't sell. Also being near Detroit helps. When times were good, back in the 50s thru early 70s, large numbers of people bought some great stereo setups. Michigan was also the home of Heathkit and Electro-Voice. Estate sales sometimes feature some really nice gear.
I think there are people out there buying/selling vintage as a business. They low ball and then over price. I may turnover gear, but I make it clear to sellers that I'm a user, not some guy trying to flip for a profit. You end up meeting some nice people.
I'm not sure what to do with the Sony V-FET amp I took out of this system. When I got I didn't even know if it worked. Except for some static in the switches it works fine. I contacted someone who has a rep for working with this type of Sony to re-condition the amp, but he had an 18 month backup.