Suggestion: Add retainers for free trial players but leave the MB inaccessible.

I understand barring access to the market board for free trial players as it can be abused, but retainers themselves, sending them out on explorations, levelling them up, as well as the added inventory space, are fun and helpful features that free trial players should have access to. The ability to use the market board could be greyed out or removed as an option entirely, but the ability to have retainers to experience the content they provide, and the added inventory space are things a free trial player should be able to experience/have access to.

You acquire the ability to hire retainers from Minfillia in the MSQ, despite actually being unable due to the free trial restriction. Retainers makes sense within the context of the story.

And finally you could sell the extra retainers (3 through 8) on the Mogstation noting that although you are still prohibited from using the market board, you can purchase additional retainers for ventures and inventory space.