Small purchase, real paperwork
Utility trailers change hands constantly on marketplace apps, and most states still require registration — which means the DMV will want proof of purchase. A one-page bill of sale with the VIN or serial number, price, and both signatures usually satisfies it.
Homemade and older trailers
Many older or homemade trailers have no title and sometimes no VIN. In that case the bill of sale becomes the primary ownership document — describe the trailer thoroughly (length, axles, color, any serial plate) so the registration office can identify it.
What to verify before you pay
If the trailer is titled in your state, ask to see the title and match the VIN. If it isn't, confirm the seller's ID matches the name they're signing. Then record the true price — registration fees and taxes are often based on it.