Hello
Sorry to be the bearer of bad news, but it's monetary value will be rather low. It looks like a low-mid range east german or russian box from maybe the 70s or 80s (at a glance). LMH tuning could be a selling point depending on where you are, but the box looks very well used, judging by the yellowing on the register switches, and if it has been stored for a long time this will have a very negative impact on the instrument.
Unlike violins, accordions do not hold their value, as technology has changed and massive improvements have been made to response and weight. Also, accordions are made of wax and leather inside, neither of which age particularly well, and their condition is never improved by typical storage conditions (loft/cellar) where temperatures can vary widely, and damp is rather common.
If you could give it a quick go and see how airtight it is, if all the keys, buttons and switches work, and whether it smells of mildew, that will enable a better assessment, but I shouldn't expect more than about £250 max if all well and good, as these low level full size boxes abound, and are generally too big for beginners and not good enough for more advanced players.
If it was down to me, I quite like an older one with a bit of character and some little quirks, but that's certainly not the general consensus. Or maybe i've just resigned myself to them because I can't afford a nicer one. Anyhow...
Your best bet would be to learn to play it - it would be worth a lot more sentimentally or in use than it would for sale.
Good luck whatever you decide, and if you're anywhere near me I'll quite happily take it off your hands - I do love a project, and it would be nice to have a spare.
James
