The other downside, and the reason a lot of the music isn't available, is the artist's contracts.
Disney doesn't own the full rights to all of the park music. For example, my friend composed sections of the Main Street Electrical Parade. While his contract states that they can use the music...
Honestly, I would start your bid at the lowest price you would sell it for and not lose sleep over. If it goes up from there (and it most likely will) then great! If not, you at least got a price you were willing to accept.
Does anyone know where I could get music from The Living Seas at Epcot? Back in the 1980's there used to be great audio tracks there, but they've never been released comercially to my knowledge.