Auction may be a good idea. Maybe a week-long (game time) auction. Some aircraft come on the market on Monday, bids are due by Sunday midnight and on Monday, the participants are informed on who won the auction and at what price.
And, players could participate by placing their aircraft in the auction.
Another problem it could solve is if a player really wants to get rid of a plane, no matter what price. When the player places the aircraft in auction, he should get some lowball bids upfront from AI leasing companies (lower than 70%), and the player should be able to set the minimum price to whatever he wants to. For example if the player really wants to get rid of a plane, he can set the price to the highest bid of the AI leasing company, which insures the plane would be sold. But real players can always bid higher. The bids should be secret, so there would not be E-bay like click-fest in the last seconds of the bidding.