I am also quite amazed by Atalanta's achievements in the last 3 seasons by always entering the Champions League spot, this actually proves
Atalanta is quite consistent in the last 3 seasons. Atalanta sold some of their players as you mentioned, it's natural to make the club's finances
stable. Because a team like Atalanta doesn't earn a lot of money, it's necessary to sell their players to help finance the club. I think Atalanta's
current success is because Gian Piero Gasperini's strategy is working very well, even though Gasperini failed to coach Inter in 2011.
But with Atalanta he was quite successful by bringing Atalanta to always finish in the top 3 for 3 consecutive seasons.
3 seasons are long and with the return of AC Milan, and AS Roma, the next season will be very challenging for Atalanta. If they will be able to end in the top 4 or the third position, they will demonstrate their powerful position in Serie A.
Success of a club is contributed by many people, coach, players, scouts, sport director, and medical team, etc. Atalanta succeeded with their small transfers like Dortmund and the players they bought at cheap price, are all playing well for them.
If you watch the match Portugal and Germany, Gosen is a player from Atalanta. They discovered him, gave him chance to develop his career and got a call for Germany squad.