This has been abused since the beginning of time...it's a never ending cat and mouse game, and big cheaters are punished appropriately.
Now that you bring it up, I will make it so the random delay on used aircraft also apply to liquidated aircraft at bankruptcy.
why not setup some sort of block to prevent buying planes from airlines linked to same forum account, also (which hopefully the forum prevents people registering with the same IP). also why not make starting aircraft have a special registration (perhaps having a letter in the end) and then making it so that if registration contains that letter after bankruptcy or lease terminations, then it doesnt get sold. Of course to make it fair, make so that if the airline owns the plane or leased it off of Used Market then they could sell it (which would indicate that they've been playing for some time because it takes more money to buy planes from lessors then starting cash allows)