Which players need to be sold, and why? Considering their performances last season, they were much better. The most important thing Arteta needs to do is build the players' confidence so they develop a winning mentality. There's a lot of hope for this squad now, as bringing in a number of quality players will significantly increase Arsenal's chances of winning the Premier League. I agree with you, as there's no need to sell some of Arsenal's players, as Arteta's core players have generally performed quite well in the previous two or three seasons.
The problem actually from my observations is that their performance keeps decreasing every season. Maybe not their performance per say, but that good football keeps missing in every new season. I think this actually reflects in some player's stats when we compare two seasons ago to now. They've come so close in three consecutive years, but is there even hope for this very coming season?. Keeping players is never a problem, but Arteta choosing not to either make substitute on time, or even play certain players, has been a very big problem.
Winning mentality needs to be in the team, but that alone won't be enough. There needs to be some sort of competition among the players. The act of favoritism should be kept aside. A player who performs well should get more playing time. This isn't time to trust the process or develop certain players. It is time to win a major trophy.