Well unfortuneately the problem is not that many people want an accordion.
Those that do either already have one, or want it for a beginner or improver.
Then there are people who want an Excelsior... Or a Soprani or a Scandalli. Or even a Hohner etc.
Or want a 4 voice, or a 72 bass to stroll with..
Nice accordion or no, it fits none of the above.
And as said 120s aren't everyones size, today either.
So if you are sure it plays well, everything works, no leaks, no smells, is in tune and cosmetically nice and are prepared to commit to that description, I think you'd maybe get £150 -300... unless someone recognises it as something that I don't.
You prob think it's worth more.. and it maybe is, but I'm sorry to say it's unlikely you'll get more.