Co-ops are groups of people who get together to purchase large enough quantities of things that they can get wholesale prices. There are lots of Yahoo groups that are coops. I really only know about the yarn and diaper-making fabrics coops, though. The coop leader typically posts info about a sale, you enter what you want in the database, the leader will announce a call for payments, and then order the stuff. When it comes in, she'll call for shipping payments and then send you your stuff.