Well, everyone gave some suggestions, but there are a few points remaining, You might know some or all of them already, but still:
You might like to RP as a very ruthless Pirate - as you call it - But always keep one thing in mind, When you go on that char, Think ooRP but Act InRP, think of yourself and the trader as players, and try to imagine how would you feel if You were the trader, not the pirate. Dont ask for something that even you yourself would get pissed off if a pirate asks you for. This is not just for you, this is for anyone with aggressive chars in direct contact with traders. Trying to act like a Real Life 'Blackheart' wont make your RP look OLOLO-Uber-Realistic-Cool, it will just make you an A with Two S running after it!
Personally, I think even asking for 200 ore from an Adv. Train with full cargo, is more than enough - But thats a personal opinion, I dont expect everyone to agree with me on that!
Want a Pirate Transport ? Check This and you'll get some nice tips about a good Weapon loadout of one, But you also have to get a Nice Armor on that thing, so you'll be even harder to kill.
Concentrate on a Single region, try to find all the ways leading to your spot and all the possible ways of going around it, if you get a trader once, change your spot, cause he might try to get around you on his way back.
Remember that you always have to make a demand before killing a trader, you cant kill him just because he doesnt Stop, so, Set some preset messages (set with /setmsg <1-9> <Msg>, and then /<1-9> to use it) for the 'less talkative' type of traders, who just like to try and run away without a single word, and make it clear for them that you will ask for their whole cargo if they try to run away (Or something like that, use your imagination) and if they dont stop, just spam a few presets and start shooting, But please, Dont use Presets to pirate a transport who actually Stops and starts to RP with you instead of going on cruise and being CDed again and again.
Cargo-Pirating can be really profitable, but remember that 'Being a A nd two S in Real Life' thing ? Don't go on your Pirate char to make Money, Go on your pirate char to get some nice RP out of Traders, don't just keep shooting until trader pays or dies, when they try to RP with you - Which is Less successful than trying to RP with a Wall in these cases.