To initiate a trade with someone, type
they will be sent a message. They then type
You will be both now in trade mode. To select items for trading, simply DROP them! press d, scroll through your list, then drop them. What will happen however is that the item 'dropped' will be placed into a virtual trade basket - so it won't actually be placed on the ground. Each of the traders will be notified as what is on offer.
When you are both done choosing items, type
When both are you have typed /dotrade - then the items will be exchanged.
You can type
at any time to abort the trade.
Once you have selecting an item for trade (by dropping it) you can't unselect it- so if you change your mind, cancel and start again. So take care selecting items for trade.