To begin a trade with a character, drag and drop an item on their avatar. The trade window will appear. It shows an area for each party to place goods in (your initial item will appear on your side). Add more items by placing them on your side of the window. The characteryou are trading with will do the same. Once both parties have finished placing their items, clicks the "accept" diamond at the top to finilize. If you wish to cancel, right-click anywhere on the trade menu. All items will be returned to your backpack.
Safe Trading Tips
While the system is very secure, there are ways that scams can happen. You should always inspect every item in the window to verify what it is. Going by the graphic is never enough and is the most common way to scam someone. A popular method of scamming is to place legitimate items in the window, you then inspect the item to see they are real, the other person "accidentally" closes the trade menu, you do the trade again but this time the scammer puts fake items in, hoping you don't reinspect them.
Another important safeguard is to examine every container. To open a container, request the owner to do so. It will display the contents to you as well. Once again, this is another common scam tatic. You inspect the container, the scammer "accidentally" closes the menu, they put in a new container that looks, has the same item count, and weight as the old one, but has fake items in it. Make sure you repeat inspections every time the trade window is closed.
Undervalued Bank Checks are another common scam. But it might also be a simple mistake when dealing with large numbers of checks. Make sure to inspect every one of them, for example a 100,000 might has been mistaken for a 1,000,000. If it is a single bank check, then it was probably a mistake. But if you discover multiple bank checks then it is most likely a scammer.
Player-owned vendors can work even better than the trade window in certain situations. For example, if you are buying a 50,000,000 gold item it would be quite a hassle to handle 50 bank checks. Buying it from a vendor allows the item seller to get all their gold without having to inspect so many bank checks. It also allows both players to transact without having to be on-line at the same time.
Drawbacks from this method include the seller needing to have a vendor. You, as the buyer, also have to fit all the gold into the bank. There is also always the possibility that someone else can purchase the item.