Never submit multi-part stories. As rare as the use of an inventory story is, multi-part inventory stories are virtually never used. Inventory stories requiring more than 21 pages or any other special format are never purchased from novice writers. Your ability to tell a story through pictures. Your ability to draw.
Comic books are a story telling medium.
Your samples should demonstrate your ability to tell a story in a sequence of pictures. Make up a story or have someone describe a few scenes from a comic book you've never looked at before and draw four to six pages of it breaking down each scene into an optimal number of panels to tell the story visually. Do not send pinup shots, since they do not show any storytelling ability. Beyond that, your samples should show your command of the human figure in a variety of activities, everything from a simple conversation to a frenetic flight scene. Concentrate on what's inside the panel borders, not on the overall design of the page.