This already exists...All ores have fences, almost all with 7k+ giving you much higher profit than reasonable allowed monetary demands when you get 3kcargo. Almost all commods also have sell points on unlawful bases.
People prefer to use bombers for convenience. You'd have to ban that or make max demand like 3 million for people to use transports for piracy.
The big issue with making transports more survivable without being able to fight back is the brutally off-putting gameplay of second new star wars movie trilogy. Trade factions and powertraders also won't cut their profit by half guaranteed when flying combat ships and losing time on them instead of trading. Player list will always be supreme for them. Instead you need to be able for transports to be able to fight back - a bomber should lose to battle transports with high armor hands down, bad bomber should lose to cap ace on a 5ker, win after a fight on equal skill level. Cruisers (the only real issue for traders - thruster is OP now) should be nerfed heavily vs transports.
EDIT: actually if battle transports could just outclass a solo bomber and give 2 an equal fight, and normal transports would give 1 bomber a good fight and win vs weaker bombers, but their cargo or part of cargo was easy to shoot off - then you got a mechanic that could actually get used and push people into cargo piracy. You would struggle to kill the trader, have no chance if they have high armour and risk dying, but to rob them and take part of their cargo if e.g 50% of hold was in the "shoot off a part" mechanic and get out wouldn't be an issue for you.
