How you you handle showing proof of ownership when buying a used dirt bike if the previous owner / seller doesn't have a title?

Doesn't seem many owners title dirt bikes although it's required.

Will just a bill of sale prove ownership?



http://www.dot.state.tx.us/drivers_vehicles/vehicle_registration/faqs.htm

Must I register/title my ATV or my off-highway use vehicle?

All-terrain vehicles (three or more wheels, equipped with a saddle or bench seat, and not designed by the manufacturer for farm or lawn use) are required to be titled, but not registered. Off-highway motorcycles (two or three wheels) are required to be titled, but not registered.