If you've got a large ship (like an Anaconda ), then one trick I've found is going around buying up multiple loads of Luxuries and Computers at Rich/Average Industrial planets, then offloading the lot in one go into one Poor Agricultural planet.
I've done several rounds of that in the north-west (ten o'clock) sector of Galaxy 2, and it's quite profitable.
Commander Ranthe: Flying the Anaconda-class transport Atomic Annie through Galaxy 2. Combat Ranking: Dangerous
"Big ships take more booty on your interstellar flights..."
), then one trick I've found is going around buying up multiple loads of Luxuries and Computers at Rich/Average Industrial planets, then offloading the lot in one go into one Poor Agricultural planet.