We'd be absolutely stupid to sell him. Granted, he's been awful this season but so have most of the team.
Like I said in another thread, he's one of the players that I'd like to see in a team/squad with a proper plan/strategy.
We can't sell everyone this summer. And even if we could, we're not going to sign 11+ replacements.
Lingard, Cavani, Mata, Pogba, Martial, Van de Beek and possibly Ronaldo will all be off in the summer. Even if some of their contributions (in terms of appearances and goals/assists) have been limited, that's a lot to replace already. We're already one player down (he who shall no longer be named).
You're also adding pressure on a youngster in Elanga to play 40 games next season.
In addition to that, you're probably also needing to sign at least one full back, a centre back and perhaps a new keeper. That's a lot to do in one window, I'm not sure Rashford is where I'd start.