We recommend E-junkie shopping cart for quite a few reasons:
1. If you are selling multiple items, having a shopping cart makes it easier for the buyer to buy multiple products in one go, thus increasing your sales.
2. If you are offering multiple payment solutions, then it's just a cleaner solution rather than having different buy now buttons.
3. E-junkie cart supports shipping, tax, inventory control, cart discounts, variable pricing and complete integration with Google analytics.
4. E-junkie cart is easy to setup for the merchant and for the buyer it provides a very slick shopping experience.
However, your conversion rate could be better with a buy now button depending on the type of product you are selling and if your buyers want to often buy multiple products from site or not.