This has been a fun discussion! When money is involved some people really get into their feelings. I enjoy the reasoned responses and the nutty emotional ones as well :)
FWIW I like the $100/yr AG Blue Book subscription and the eBay completed sales feature as my guide as I'm assembling another system on a budget. "Budget" is a relative term of course- everyone's got a different one.
Anecdotal observation: I was given a ARC VT130 that was blowing screening resistors. It was a mystery gremlin as the 6550s were red plating, the screening resistor was repeatedly blowing with no shorted tube, no shorts in the circuit and no failed transformer. I called ARC to get a quote on shipping and repairing it. I was told they had an 8 month waiting list for service!!
The fellow there told me that during COVID people were digging their old gear out of storage and reinstating it. Stored units deteriorate and electronic parts degrade, pots get corrosion, etc.
Moral of the story: 1) buyer beware. 2) leave room in your calculations for repairs. 3) Figure out where to have the gear checked out and how long you will have to wait to have it functional.
Thanks for all the fun input!