You can write a phrase down like "My vouches" and highlight it. Then click the little button on the top of the post with the world and chain link picture. It will pull up a window you can add the link of your profile into. Add the link and hit ok. Then save your profile.
After you've completed a trade, someone will "vouch" for you. You hit the "quote" button and then copy the the sentence that is given. Go to control panel. Paste this into your user profile section. That's how you add a vouch.