elements:
Silver, gold, iron, hydrogen, etc...
used in manufacturing
molecules: Plastics, water, etc.
used in manufacturing and consumption.
Also, finished products, art, rugs, baskets, etc... anything that is handmade by artists.
Also, high quality goods. Computers, guns, etc.
Just like in the US today I can buy an american made smith and wessen, an israeli desert eagle, a russian AK47 (well I could if machine guns were legal here

).
Foods could also be a good possibility. Different planets offer different climates for different foods to grow. Not to mention the local government might have restrictions on exporting livestock or whatever to prevent loss of revenue. (you can export Xxzort steaks, but not live Xxzorts which can be used to start herds on other planets... cause thats loss of revenue for planet momlambo)