Not knowing what exactly you are thinking of selling it is difficult to give exact advice. One of the things you have to take in consideration is postage. You need to do research, which way will be best to post. If your items are small then offering combined posting attracts buyers, or list some items in set of 2 or more.
As sparklez said, you must not list more than one of any particular item or eBay will remove your listing and you will have violation recorded against you. But you can sell variations; for example if you sell T-shirts you can list red one in size 10, and another red in size 12, and you can also list yellow size 10 etc.