There are three requirements to be listed:
Site must list products.
Site must list prices.
Visitor must be able to purchase via mail, email, phone, fax, and online shopping.
Sites that submit individual pages, parts of a site which is not the main page, and re-directs will also not be listed.
Popups, missing graphics, broken links, ect. , under construction pages will reduce your chances of being listed.
Sites consisting largely of affiliate links should not be submitted.
For more info, please review: http://dmoz.org/add.html