If the trader you're trying to get to give you money isn't, then you are doing something wrong.
Pirates aren't nice, pirates aren't pleasant. A nice and pleasant pirate won't ever get a credit from anyone.
If every trader in the system a pirate is in doesn't run for the nearest exit the second they notice the pirate is in-system, the pirate isn't doing something right.
Traders are basically exempt from the pvp rules for a reason. Although I don't think the original reason was to help pirates, that is how pirates should use it.
Instead of trying to pirate every single trader you see, just pick one, and keep demanding money until he pays. Heck, double the payment every time you have to kill him.
Or torture him. Shred his ship until almost dying, then let him do his run without trade lanes or cruise. I wish I thought of that one, but it is something pirates should do.
Look, to pirate right you are going to need to ruin someone's night. Then every time they see you they will pay up.
If a trader isn't scared to death of you, then he prolly won't pay you very easily. However if he knows you will destroy any chance he has of making money, he will pay you quite fast.
Hmm, where have I come? Why does it seem so familiar to this one?