For future reference, is there anything needed when you sell your drone after registering it? If something were to happen, the serial on the drone may be still linked with your name under registration.
There is nothing to do if you aren't flying for commercial purposes -- since that does not require registering the aircraft itself. If you did register the aircraft with the serial number, then you should deregister it. Contact the FAA for help. Or, perhaps someone in the know here will explain how that process works.
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.
If it was registered "Commercially" (otherwise the Operator is registered and you simply remove the Hobby Registration # from the hobby aircraft) you'll want to contact the Aircraft Registry in Oklahoma City and request that the aircraft be de-registered.
Their contact information is shown below:
Toll Free Long Distance (866) 762-9434
International & Local Oklahoma City Area (405) 954-3116
They are open from 7:30 a.m. to 3:30 p.m. Central Time
For U.S. Postal Service, Regular and Priority Mail:
FAA Aircraft Registration Branch, AFS-750
P.O. Box 25504
Oklahoma City, OK 73125-0504