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.
Martial's arrival is just to fill the position of Youssef En-Nesyri who will be played for Moroccan national team. Based on rumors circulating, if Sevilla's offer successful, the most likely option is to only bring in as a loan player. Maybe, if En-Nesyri returns to Sevilla, Martial will be on the bench, unless he can show a good game, sevilla will change their decision, and may be an option for Sevilla to buy Martial for permanent.