There's a healthy market for used planes.

Allegiant is a great example. They fly slightly less fuel efficient MD-80s which can be bought at bargain prices from the legacy carriers (MD-80's are those narrow body jets with the engines in the back and the 3-2 seating arrangement). They make it work by flying nonstop flights from smaller airports (without competing carriers on those routes), with low frequency. The low frequency also gives them longer until they have to do a heavy maintenance check -- which, for an older MD-80, usually costs more than the plane is worth. http://www.fastcompany.com/1325762/heard-allegiant-air-why-i... .