Traders won't pay tax if you destroy them from the moment u see them as some pirates do. U need to convince them that it's better to pay tax than loose their cargo. U also need to know how much profit they make on that route and adjust the tax accordingly.
I have a suggestion that might improve both Pirate and Police RP. Can we figure out a counter that counts how many taxes/fines has a char received? Something to give pirates/police a bit more incentive to figure ways to tax/fine instead of just shoot?