You can buy, sell or post with just 1 ID on any of the eBay sites. If you are going to sell regularly, you might want to have 1 ID for buying and 1 for selling. If you are just starting out though, you might want to have just the one ID for now so that you can build up your feedback.
Some people like to use a posting ID just for posting on the forums though I'm not a big fan of that.