I still cant understand why is not possible to unload cargo from ship to some station hangar? Why is possible to store 30 ships with 1000T mass for each and is not possible to store some cargo at station or planet?
That is because Commodity trading affects the BGS and ship purchase doesn't.
Fdev wont allow you to stock pile good in stations which could then un-balance a faction's influence because of a sudden flood of (panic) selling.
Storing commodities will also make you ship lighter, allowing you to travel further per jump so if you purchased 200t of cargo, stored it, jumped to CG system and then recovered the cargo and sold it, the economics of trade would be undermined.