Right, if you actually registered the aircraft as for Part 107 use, write to the Aircraft registration Branch of the FAA requesting that it be deregistered and state the reason is that his has been sold, and to whom. The new owner can then register it as he needs to. If he wants to register it for commercial use, h may need a bill of sale from you to show transfer of ownership. Again, this only applies to aircraft registered for commercial use. If you have been using it for hobby flying, just take your registration off before you sale it.