Make and show some prototypes, and the rest of the selling job ought to be easy. I once made a trailer for an organic bakery. They used it every morning to bring their flour from the mill to the bakery, and then to deliver the bread to the stores in the afternoon. Lots of people saw that trailer, and I had several more orders because of that one trailer. If necessary, you can even make trailers at low-cost or give them away to certain agencies or individuals if you think the visibility this brings will help your business.
Bicycle trailers are easy to design.
They must be as low as possible so they do not tip over. They should be fairly narrow to negotiate traffic safely, but not so narrow that they are tippy. Because weight is a consideration, you'll want to design them with no suspension or lightweight suspension. Suspension is a good idea, because unloaded trailers do get very tippy when hitting bumps. The trailers should hook to a spot as low as possible on the bicycle, but not behind the bike.