Generally if you have good players at the start hold them until the game has been out for a week to maximise profit on selling. However if you are wanting to sell to build a strong team remember that all other players will be high in value too. So the best way is to sell them all and then build a lower rated team grinding for packs etc. This advice is aimed at the general FIFA player not higher level players who will desperately need crazy teams to compete in the top divisions. Great video from FIFA trader FUT_Accountant on Twitter covers these points.