For purchases of vehicles newer than 15 years of age, you will need to present a certificate of origin, a title or a title application prepared by a NH dealer. 

For vehicles 15 years of age and older, a bill of sale is required along with a previously issued current or expired New Hampshire registration, a valid New Hampshire or out-of-state title (or copy), OR a completed VIN Verification Form 19A (available at the Town Clerk’s office). Both the NH or out-of-state title with completed transfer of ownership on the back is acceptable as a bill of sale. If you wish to transfer your current NH license plates you must present your existing registration so that a determination as to whether or not the plates are transferable can be made.

With all transactions you will need to show proof of residency (current utility bill, closing statement on home, lease agreement, etc) and/or proof of identification (driver's license).