How do I have products show up on Category pages?

I would like for products to show up on my category and sub-category pages.

For Example: You click on "Other TCG/CCG/CG" category. I want all of the products that are within this category to show below all of the sub-categories show on the top. Then if someone were to click to view one of those sub-categories, lets say "Dragonball (Bandai)" there as well I want all of the products that are in those sub-categories to show up below them.

Also I would like the products not to be duplicated but basically the same listing just shown if two or three places. Lets say there are two listings of the same product. Each one says 3 are available and someone purchases one item from one of the product listings. Both of the products should now have subtracted one from the total available.

Anyone know how to do this? I have seen some zen cart sites that have it so I know it is possible but I can't see to find the answer on my own.

http://www.schobookcollectibles.com/index.php?main_page=index&cPath=65

Thank you,