what it's worth, where to sell it, etc.
I have been able to find a few tenors and a few single valves but I haven't seen any other double valve bass trombones.
The serial number is 238xxx which I think dates it to 1956-1958.
It is in pretty good shape overall - slide works well, just a few small dents, finish mostly intact.

https://www.dropbox.com/s/c9kiskwsf99jv ... e.png?dl=0