If you don't want your aircraft anymore or you just need the money then you can sell, lease, or scrap it by clicking on the $-button on the fleets page.
If you want to put it on the traders list you have to first choose a price (something between 50% and 100% of the aircraft value). Then you can choose if you want your offer to be visible to just one certain airline or everbody.
If you decide to lease your aircraft you have a lot of possibilities for the leasing contract.
The money you get with the first payment
The money you get every week during the leasing period
Here you can enter the leasing period. When the period is over the aircraft will be automatically returned to your fleet.
Enter an airport where you want the aircraft to be returned to after the leasing.
Add a deposit which is fully refunded by your airline after the leasing.
Must repair if status below
Choose a minimum status where the leaser has to repair the aircraft.
Same as when selling an aircraft. You can choose if you want the offer to be visible to only a certain airline or everybody.
Not much you can do there. If you accept the price offered for the scrapped parts you can scrap your aircraft here.