Display Interactive Item in Posts

Yep maffa50's hit the nail on the head. You can link items from your stash by left clicking your characters portrait in the top left corner of the page and finding the item in your Stash. Left clicking an item while you've got the New Thread page up and Content text box selected will link it (a code in square brackets should be pasted in the text box).

Just a note, if you link an item on the trade subforums, these will show up on trade searches. Keep this in mind if you'd like to share your items in a shop thread for example.