Scrap is much better than sell in Private Worlds(because there are less players than in Public).
In Public Worlds it may be good sell the aircrafts but in private worlds i didn't se good revenue trying to sell aircrafts.
Scrapping them will be, around, 7 or 8% of it's value(in the date of the scrap).
Example: Today I scrapped one Bac 1-11-200. Age: 5,0 years, value: around 27 million. I Received 2.2 million for the scrap, in just one click.