Can You Sell Besra?

No, you cannot sell the Besra. Unfortunately in GTA Online it's not possible to sell Special Vehicles, Pegasus or Facility Vehicles.

Can you sell Pegasus vehicles in GTA?

Once a Pegasus vehicle has been purchased, it cannot be sold or removed from the list of purchased Pegasus vehicles.

Can you sell a Pegasus aircraft?

pegasus vehicles can't be sold in any way. you can sell smuggler's run aircrafts in the hangar (other than the lazer which released in smuggler's run but is a pegasus aircraft).