Is it permissible for a Muslim to bring foreign traders who sell both lawful and unlawful products to participate in a trade exhibition, with the aim of helping the organizers set it up, in exchange for him receiving free spaces therein to use for his Islamic Da'wah?

It is not permissible to grant permission to one who sells prohibited items to participate in the exhibition, and a good intention does not justify a prohibited means. As for one who sells permissible items, there is no objection to granting him permission. It is also permissible to assist the exhibition organizers by bringing in vendors for a commission, even if it involves the benefit of some spaces within the exhibition.