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This happened because they were too quick to sell players just because they wanted the benefits they had which resulted in De Zerbi inevitably becoming a victim to be fired by Brighton before.
Brighton had always been one of the clubs that produced stars and they became one of the dark horse clubs that were very wary in the De Zerbi era but mistakes in transfers that released too many star players such as Trosard, Caicedo, MacAllister, Bisouma, Cucurella or even Ben White actually became the beginning of destruction where they increasingly lost momentum because their core players left and there was no good backup as a replacement.
Now they are back to trying from scratch but of course it takes time so for this season I don't think we can expect much more from Brighton as it would be great for them to be in the top 10 as well.
that's true, and that's the main reason why De Zerbi resigned from Brighton, he avoided dismissal. The transfer policy taken by the management is likely to be the trigger for the manager to decide to resign, it is clearly not in line with his plan to build the team. However, most mid-table teams maintain their existence with a business model of buying and selling talented players in every transfer window.
De Zerbi didn't resigned from Brighton because of the sale of players that Brighton did in the past. Brighton has always been a selling club even before De Zerbi was appointed. De Zerbi saw himself a an improved manager who shouldn't be managing a mid table club, whose ambition was just to survive the premier League without the ambition of becoming a power house in the premier League. De Zerbi was heavily linked with the move to Chelsea and because of this he refused to extend his stay at Brighton, but he was eventually betrayed by the Chelsea management when Enzo Maresca came into the picture. With De Zerbi already informed Brighton that he won't stay further, it was difficult for him to take back the Brighton job when Chelsea snubbed him for Maresca, that was why he went to Marseille. It was because of the Chelsea job that made him resigned his job at Brighton not necessarily that Brighton were selling off their best talents.