Wow, depends on how you play i guess, for me it's usually more buying than selling (except for long games on big maps, with a big trade network you're always buying)

This is probably a stupid thing to ask, but can't you also make multiple trade ports per planet? It never occurred to me until someone mentioned it in another thread, since in every other 4X I've played you can only build one of something on each planet.

So... yeah, research colony size improvements and trade ship cargo capacity. Also, I didn't realize this until my 5th or 6th game, but culture is EXTREMELY important once your empire begins to expand. If you've got a planet more than a few jumps from your homeworld, its income begins to drop dramatically! If you build lots of culture stations, the loyalty goes up and instead of getting 0.2 of metal you might get 0.6.

If you had 5 or 6 trade stations on your main planets, you'd have a juggernaut economy.