The current trade system where you buy/sell goods to space stations and load/unload goods from planets you own is difficult to use at best. I am reworking the system to be much easier to use and the following screen-shot shows an early version.
Every trade good you own is listed with a quantity bar next to it and every trade good available on the planet or space station is also listed with a quantity bar. When you move the bar for items you own, the value you will be paid for the goods at a station shows (if you are at a planet, no value is shown.) When you move the bar for items on a planet or station, the amount of usable space in your personal craft adjusts accordingly and if at a station, the price to purchase the items also shows.
The new interface will make buying, selling, loading, and unloading much easier and will be available in the next release!