As a guideline, players that seem that they will not be able to progress enough in next season to meet the quality requirements should normally be sold. But take into consideration, that you must still keep a decent amount of players for playing the first few matches next season, especially as a precaution in case you will not be able to buy needed players early enough. Despite having tokens and virtual money for transfers, the needed players or the players you will be looking for in the market may not always be listed. Also take into consideration that injuries may occur early in the season, and thus you will have to keep some backup players for some of the key positions.
But, this does not mean you have to sell your players. You can keep most of them for another season. It all depends on what your opponents will have in the next season.
By the way, for your region, it most likely that you must avoid afternoons and evenings (especially in the first week including first Sunday and Monday) for bidding in the transfer market. Best is in the morning.
I leave the rest for you to figure out.