I used my seeder/spreader for salt in the winter, and now the main wheel is
bound up. How do I fix this?

I tried to find info on your spreader but had no luck.

If the device has any metal parts & you did not rinse it immediately
after the salt use; you've got a corrosion problem.

If it's all plastic try hosing it out.

Other wise you need to disassemble, clean up, lube & re-assemble.

cheers
Bob

You can remove the wheels and pull out the axil then lube with WD40.