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.