First of get yourself a trade membership on here as some people have an issue if your a company and just join as a member, very cheap so shouldnt be an issue.
Some feedback on my experience with trailers.
If you have a boat in a dry stack and also have a trailer there comes a time when the question comes up whether its worth keeping the trailer for the one maybe two times a year you might use it, plus it just sits there and starts to disintergrate in our weather, so maybe a good idea would be to have a number of different size trailers you could hire out to people.
Light boards on trailers in my opinion are really poor in terms of design and dont last, perhaps design a more robust and weather proof version of a light board. People would pay extra for a well designed light board.
Spares, create a on-line decent spares capability so people can order parts for their trailers. Hunting around for spares can be an issue and if you turn up in person many times trailer shops dont have the bit you want.
Perhaps sell second hand trailers also.
Many people unsure of the law and trailers, any guidance on your site would be useful to people.