tvad,
I would second Audio Asylum as well. Ads are free as well.
Vintage 1960s Amperex and Siemens 6922/7308/E88CC/E188CC are still in demand and come at a premium price.
As for reference pricing you might check out Brent Jessee’s website.
http://www.audiotubes.com/6dj8.htm
I assume you still have a tube tester so you can list the tubes test results.
Jim