I have a copy of The Wall on Betamax which I would like to sell. Does anyone know how much I should be asking and where to try?

No idea but, I actually have one as well as VHS, wow, does anyone have a working Beta? LOL, I even have quite a few other older concert Beta's, if ya sell it and get a lot let me know!

If you're looking to sell, I'd try Ebay. What you should do is see what comparable items are going for to get an idea of where to price it, if that's the route you decide to. Of course, you need to bear in mind, Betamax players are about as available as 8-track players, so you're selling to a very, very focused group of buyers.