How does selling stuff work?

[Jorlem]

This has been bugging me for a while: when I sell something, it vanishes from my base, and the passing spaceship sends down drop pods with silver or whatever I traded for.  But how does my stuff get up to the ship?  The method, whatever it is, obviously works on humans, as the pirates can "beam up" slaves you sell, so why can't I build up enough money and purchase passage on a passing ship?

[nmid]

They don't want to lose their market and want to keep you on the planet, so that they can keep coming and trading with you.