you would need an ssl if you are selling products from your site and are looking to integrate with some form of shopping cart program, if you have an ssl certificate it secures the transaction from the buyers browser to your server, a dedicated ip address is also needed if looking to use a ssl certificate, an alternative would be to use paypal if only selling a few products as then you would only be linking to an order form and the ssl would be on their end, if selling tons of products and wanting to set up your own shopping cart system getting your own ssl & dedicated ip is the way to go, basically it secures or encrypts the transactions and makes them secure from the buyers browser to your server.