If you forced traders to re-buy the ship, some, but not all pirates would intentionally destroy the trader or make 20-30 mill demands. (Better then losing a 150 mill ship) Imagine;
Trader: Ahhh! Trident!
Pirate: Pay 20 mill 5 seconds. [/givecash command typing would take longer than five seconds, considering the pirate's name is MyLTHs(RUS)]
Trader: Hold on, let's talk about this. (Five seconds of typing)