Close all routes from the plane you wish to sell. Go to Airplanes --> View and Buy and, to the right of each plane, there will be a link titled "Info". Follow this link. On the next page, right column, there will be two links: one to sell the plane, another to offer it for leasing.
If you had not closed all routes, instead of selling and leasing there will be a link to close all routes that plane is flying.
Once you press the "offer this plane for sale" (or similar text) link, you'll have to fill a form:
* ID of the airline you're selling to - leave this blank or at zero to offer the plane on the open market
* Asking price - input a value that is at least half the value of the plane (this is shown) and no more than 133%. Do not enter commas, just digits.
