When you reach Jr VIP or Marketplace seller status, you will be able to have a signature. Your signature shows on the Information tab of your profile and directly underneath the first post you contribute to any thread.
You must be an upgraded member of the forum before you will be allowed to use a signature. You may post 2-3 lines of text or 1 banner, 728x90 pixels max. Do not use a font size larger than 2, crazy fonts/colors, sound clips, excessive smileys, or anything else annoying.