The primary place of item exchange is the market, where every supply and demand information is available. There is also another way to do business between two Agents, the so-called direct trading.
Since there is no common interface, direct trading can only be started by an Agent who sends a trade request to another Agent. If you want to send a request, just right-click on the name of another Agent and choose Trade.
Each participants have to be on the same terminal or outpost to perform direct trading.
Direct trade request are accepted by default, therefore a trading window can pop up on your screen at any time. If you don't want to be disturbed, you can disable the automatic acceptance of trade requests on your Agent info page.
In case the request has been accepted by the other Agent, the trading window will open, where both participants can make their offers. The left side shows your offers, the right side shows the counteroffer. Money can be added by entering the amount in the bracket, but the items need to be added manually. To do this, open a storage of yours and just drag the item into the trading window. When you are finished placing your offers, click on the send offer button to send it. Every offer can be changed until both of you press the Accept button, that finalizes the agreement.
Only those items can be traded that you can package. A damaged item, for instance, cannot be traded.