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.
Any manager that aims at going higher would do same, Brighton as a club don't have the spending force to sign players that would compete well in top competitions in case they qualify for it but would rather buy young talents at cheaper price train them into prospects and sell them off to top clubs and that's why they settle for midtable. Zebri is a fast developing manager with good tactics he's someone that most top clubs would be looking up to when their current managers resigns or get sacked due to poor performance. Well, I didn't expect him to leave England cause I felt either Chelsea or Manchester United would sign him but he moved to a good club in the Ligue 1 and his team is doing fine over there. Well their's hope for a return to the EPL in future and I believe by then he might have won a trophy in the Ligue 1 and gained more experience over there.