Alright so I have decided to sell the spark, and upgrade to something else. Is there anything I need to configure or reset? I suppose the spark is registered to me and my phone in some way?
The Spark is registered to your DJI account. There is no way to unregister it. The new owner just needs to install DJI GO, create a DJI account (if he/she doesn't have one), and log into that account within DJI GO.