Somebody plans an order from a wholesaler (cheaper per unit price on a good) and multiple people opt into the co-op by contributing a dollar amount usually defined by the price per unit of the good, when the order is placed the person will distribute the goods to whoever opted in.
Say you want X
mod but Wholesaler only sells them in orders of 500+ now per
mod you can get them from this guy at half price but you don't want 500 of the
mod so you get 499 other people to help pay for the order.