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These players were flop and they have no contribution for Manchester United for last season so it's time for them to leave the club and selling them is a good option because Manchester United can buy new players using those money but Manchester United have to realize that there are no clubs who willing to pay with expensive price to buy them so if Manchester United want these players to sold i think they should be sell them into cheap price
Selling the players they currently have when their performance is declining will be a big loss for Manchester United, because the value of their players will also decrease. But keeping them is also not the right solution, Man United clearly needs a refresh in their squad. So whether they like it or not, Man United must sell their players at a loss and spend their money again to be able to recruit new players who are more promising.
You are right, there is no way a team will buy a player high amount knowing fully well that the team they are buying from is underated and some team won't be even interested or willing to buy from those kind of team. Indeed Manchester united really need a new team entirely even though it will take them longer time to come up but I still think no matter how they try, they can never be as good as before. One funny thing there is that Manchester united has the money to buy as many players as they want but the problem now is not about buying because the ones they have been buying has nothing to offer or has never offered anything.