Problem is an unintended, but foreseeable, side-effect of the minimum pricing being changed due to regular cheating.
That flavour of cheating relies on co-ordinating between buyer & seller, so buyer knows exactly when to refresh the market and grab the publicly listed plane.
Simple solution, which I put in a different thread somewhere, is to stop publicly listed planes appearing immediately. If you list a plane publicly, if you swap a plane's listing from private to public, if you adjust the price an a public listing, then in each case the plane should then not be viewable/should disappear from market, should randomly show up in any of the next ~5 UM restockings/next 24 hours of real time.
That way the minimum prices for public listings can be restored, so players can compete with/undercut brokers on pricing of unpopular models and actually sell some planes. Private listing price restrictions can stay as they are, and people can no longer get around alliance/private price restrictions by colluding about listing times.
If you want to sell your plane to a specific person, that's fine, abide by the strict restrictions on pricing. If you just want to sell it for less than the brokers are, that's fine, you'll have a wait of up to 24 hours and can't control who buys it. If you want to sell your planes even cheaper than the cheap broker price to a specific person, then stop trying to cheat. Easy.
And I think it'd be relatively simple to implement technically.