I would be very careful about trying to market it. If (as seems likely) you don't have the copyright to it, you could well be sued by Disney for being in possession of it. I know that the BBC have threatened to sue people for holding episodes of Dr Who that the BBC had lost, even when they offered to give it up for free - and Disney are more ruthlessly capitalistic than the Beeb!
There shouldn't be any legal problem selling it- Disney (along with most other companies) released many of their films on 16mm and 8mm format for sale. But I've got no idea of who you could sell it to or what you'd get for it. (Incidentally, "Steamboat Willie" was made in 1928- unless there was a remake- and it was Disney's first sound film.)
My dads got a copy of this on 16mm. 16mm was the video (or DVD) of its day. While never as wide spread as video became there were plenty of films released on the format. It might be worth a few bob it might not, as its 1946 its unlikely to be a sort of first issue thing so I would imagine its not going to be enough for you to retire on.
There are film collector's fairs in London ( Ealing Town Hall ), Blackpool and, I think, Liverpool. It may be worth anything from �35 upward, as WoWo says, not enough to retire on.
The BBC does not sue anyone for having a copy of an old programme, they are keen to recover them. All they do is make a copy and return the film print to the owner, as has happened with a few Doctor Who episodes recently.