![:P](http://www.airlineempires.org/public/style_emoticons/default/tongue_reg.gif)
The game, I think, has all of the basics set up to cope with such a system; the aircraft detail pages tell you who owns the plane, who owned it before, and what it's worth.
This would sort of act as a balance to the used plane market.
Let's say I have a nice shiny new Trident 3 (which I do, in 3.1 Realistic). I like the plane quite a lot, and it can be profitable on shorter flights. But it's currently sitting on the ground, doing nothing (because I don't really have anywhere to send it). I bought it at full price, because I vastly overestimated the number of flights I needed to run. So now, it's just sitting on the tarmac (or in an aircraft hangar) costing me $308,000 per month in maintenance fees, etc.
If I sell to the Used Plane market, the broker takes off a fee of 5%, but sells for full price. When someone else wants to buy the plane, they pay full price + 5%.
But let's say that there's a different system. Let's say I jump onto the AE3.1 forum, into a special "Looking to Buy a Plane?" forum, where I can post a topic saying, "I've got this plane, here's what I'm willing to sell it for." Then, if someone wants to buy it, they can make me an offer (or it could be first come, first serve).
Cuts out the middle man, smaller airlines can get good deals on planes that larger airlines don't need anymore, and everyone goes home happy. Well, except maybe the used plane brokerage.