The pirate transport doesnt have quite enough firepower, it's do-able, but you still have a lot of cargo space even in a rogue gunboat, which has better success pirating.
And as far as I know, there's nothing against taking someone's cargo and selling it, though the destination you have to be careful about... you cant very well pirate someone right outside new london when that was their destination, and then sell the goods you pick up at new london... because it's a lawful base, and you're a pirate =P
I sometimes demand cargo instead of money if the target is carrying something good... hfuel, ship plating, plutonium, diamonds, etc.