I don't know if it is a good decision or not, but he needs more chances to play. If he always sits on the bench, it will be bad for his career. The problem is Man United has already too many attackers now, especially they have CR7 as the regular player for CF position. In this matter, I don't see any chance for Martial to compete with CR7. Also, Man United also has Cavani in the same position. However, both CR7 and Cavani are old players, probably won't last in the next 2 seasons. Man United only has Martial as a young CF there.
Since joining Man United, Anthony Martial can't give his best for the team, and maybe that's why he's always on the bench, so selling Anthony Martial is a good option for Man United. Sevilla and Juventus were reportedly interested in signing Martial and I think this is a win-win solution for Man United and Martial, where Man United can reduce their number of attackers and Martial has the opportunity to play regularly.