I think this has to do with "titling" requirements of over-the-road vehicles. For most of us we get a title when we purchase a new car, or else we get the title signed over if we're buying used (including trailers). But for a trailer, and I'm guessing especially purchased from out of state, you instead get a "source of origin" document to validates initial ownership. I think if you buy a trailer from an in-state dealer you'll probably get a title. "Bill of Sale" doesn't prove who originally owned it; DMV wants to know it wasn't stolen.