To sell stuff in Starfield, go to any store or vendor in major cities, settlements, or spaceports. Speak to the vendor and ask to see their items for sale. Then, press Q (PC) or LB (Xbox) to switch to the selling menu. From there, select the items from your inventory or your ship's cargo that you want to sell and confirm the sale. You can also use Trade Authority kiosks at major spaceports, which allow selling items directly from your ship without carrying them in. Some vendors specialize in certain types of items, so general merchants such as Jemison Mercantile in New Atlantis are good for selling a variety of goods. Stolen items require specific merchants or Trade Authority locations to sell. Additionally, investing in the Commerce skill can improve sale prices and discounts when buying.