You can't trade with NPCs. You can purchase a few items directly from an NPC shopkeeper if you enter the NPC shop in your starting city (note that these shops only exist on race starting cities, and not in faction starting cities).
Otherwise, you can purchase from the "NPC market" once you have enough credits. This can be done by using the Market link on your right menu.
Other than that you generally need to trade directly with other players, and cannot purchase anything else from NPCs. A guide to trading is located in the Guide, linked at the top of each page on the Combine's website.