The axle must be strong, and therefore heavy, in order to avoid bending. A better design may be to use a fork design to mount the wheels. This way, you can use regular bicycle hubs, and your trailer is likely to have a better strength to weight ratio. Custom trailers sell for more money than you might realize. Industrial customers such as airports, restaurants, delivery services, and large manufacturing facilities save a lot of money when they can take an engine vehicle out of service and replace it with a people-powered vehicle.
Good trailers start around at least four hundred dollars.
Unlike custom bicycles, the welding or brazing does not have to be filed to perfection, and the paint is not expected to be bullet-proof. The customers know what the trailers are going to look like in a few months, and generally wouldn't want to pay for such frills anyway. So far, I have talked about the frame and general design of trailers. If you can design a nice cargo holder, that will make a big difference to your customers. A flat bed trailer in which the user must tie in cardboard boxes is not nearly as elegant as one with nylon, wooden or metal sides.