I must admit, this is where I'll struggle when it comes to moving to a boat. I can't resist guitars and currently have <counts on fingers> six electrics, two amplifiers, an acoustic and an electric mandolin. I know some will have to go, but I can't think which ...
As for other stuff, I'm in the process of Kindlerising all my books. My music is mostly digital these days, stored on an Ipod and a Brennan, and my vinyl will be Ebayed as soon as I get around to it. Like others on here, I'll be going through my automotive tools and ebaying about half of them. Other stuff will be charity shopped: we took bags and bags of stuff and gave it away last year and it's surprising how cathartic giving stuff away is! Furniture will be sold or charity shopped. We'll keep a bicycle each, from our collection of, er, six. One of the reasons I want to move onto a boat is to simplify things a bit ... as Jerome K Jerome said, the trick is not to think what you could do with, but to think what you can't do without.