What kind of business are you planning on putting up? Have you investigated the costs associated with a merchant account of your own, especially chargebacks? Have you looked in to using a third party processor and the costs associated with them?
What you will need for customers to buy through your site is a shopping cart and a secure server (this is a MUST). The shopping cart should give you everything you need to sell on line. You will need to list the items you will be selling and a "buy now" or "add to cart" button. You will need all of their credit card information, assuming your primary method of payment will be credit cards (i.e. name on the card, card number, card type, expiration date of the card and in some cases, the verification code on the back of the card.
Hope this is of some help,
Vance