Is it permissible for someone to sell sefarim (e.g., a gemara) to non-Jews?
Does it make a difference if they are messianic "Jews" (people who believe Jesus is Moshiach)?
You definitely need to ask a rav. It is possible this is permitted in case of non-Jews who will handle the sefarim with respect but likely this is NOT permitted in case of messianic Jews.
Two relevant sources you might consider when asking are
The latter could be exactly the scenario of Messianic Jews using these gemarot to spiritually endanger other Jews.
Again, CYLOR before acting on what you read here.