I do find it odd that Pirates also have black markets on their worlds/stations. How does that even work, I wonder? If nothing else it would make sense for pirates, at the very least, to have lower tariffs than everyone else because their default market (should) work just like a black market elsewhere, sans the rep loss for big sales there.
A more reasonable take on tariffs elsewhere, however, would be to have them scale to your reputation. Meaning if they don't like you, or don't know you, then yeah, 30%+ is the going rate. But once your a commissioned ally who's proven themselves... well, perhaps a employee/friend discount is in order, yes?
Something like this:
-100 to -20 = 45%
-19 to 0 = 30%
0 to 75 = 25%
76 to 99 = 20%
100 = 15%
So you might be able to squeeze a buck out of trading, if you're lucky and are on excellent terms with both parties. However, as a caveat, I would also propose increasing the restrictions on what goods are even allowed in the trade markets to begin with - and link those to reputation the same way faction military stores are. Meaning people who don't like you at least a little bit aren't going to offer you weapons, marines or military grade ships, period. And what do you need that heavy equipment for, bub?
(For more on that, see Star Traders Frontiers and Trade Permits)