They could force you to re-sell your tickets through the team MLB site, and to sell them for face value.

If the tickets come in at less than face value because of the season sale (not unreasonable), that can work OK (particularly for good seats for a team like the Dodgers). Most folks simply won't be able to sell all of the tickets. The goal isn't to make ad hoc ticket sales a necessarily profitable enterprise, the goal is to sell season seats, so you have to be somewhat accommodating. Pretty hard for anyone to go to all 81 homes games.

This can only go so far, unless you make the sold ticket not transferable.

They can also allow some margin to be just outright sold at market. I know several season ticket holders who sell the tickets to the big games (like Dodgers/Yankees) at a premium to help offset the entire season ticket package.