Here in Michigan we can also register a trailer as a homebuilt. A cargo trailer has to be weighed, and meet a few requirements, but a boat trailer essentially just needs lights and depending on load, brakes.
Recent (couple years maybe) changes in licensing have caused a huge influx of homebuilt trailers because it's easier in many cases to get it licensed that way than to jump through all the hoops to get a manufactured trailer legal.
'Odd' story, I was trying to re-register my old boat trailer that hadn't been used in a couple years, had my old registration and bill of sale from several years prior, along with a few other documents proving ownership, and still couldn't get a tag for it.
My friend sold a trailer and I had delivered it a few days before and I still had the plate in my truck. Asked them about registering that plate to it,and then transferring it to my name, and they said no problem.
In short, I was able to register his expired plate and transfer it to my name for less hassle and cost than I could re-register the trailer to myself outright.
Ken.