Just chatting with the seller. Asking him if he'd like me to post a thread here as he bought it at a car boot and has absolutely no idea how or if it works!
I did a quick Google and came up with @helen explaining about this "mythical" model, here: The Mythical Selmer Mark VII Baritone Saxophone
So I suggested to the seller some of you may have ideas on value, or want to ask questions about it so as to help him with his sale. I've suggested he may like to donate to the forum charity if we do help him to sell it.
I have absolutely no idea what it might be worth. More photos here: Selmer MkVII Bari
It looks to me as though a buyer would need to factor in up to a full overhaul as it's not in pristine condition, but that's my only comment. Over to you... in a nutshell... what's it worth?
NB. I have no connection with the seller whatsoever.