Trade Goods are unlocked by researching certain techs. Xinathium Hull Plating, for example, is unlocked by Advanced Hulls. You still have to build them, however, before they have any effect. If the AI beats you to one, you can still trade for it (or conquer the planet it is build on). If memory serves, there is a bug with Gravity Accelerators, that prevent you from getting the speed bonus, if you trade for it. I'm not totally sure, however, if the bug happens with this Trade Good or another.