Back when I was building trailers for gliders, the local MVD office let me declare a value of the trailer for registration Then, after years of allowing a simple declaration of value, they changed to a requirement that made me present all of the invoices for construction. Steel, aluminum, rivets, nuts & bolts, wheels, tires, wiring, lights, hitch, and everything else had to be itemized. Needless to say, this was a major pain in the rear. It was about then that I decided that this requirement was the last factor in whether or not to continue building trailers. (Mostly I was tired of putting out that much work for not a great rate of return. All I was doing was delaying starvation- not eliminating the possibility.) Before considering whether to claim a factory trailer as a "homebuilt," make sure your state MVD doesn't have ridiculous demands like this.