Almost any website can be converted to include a shopping cart. It could be a WordPress blog or even a static html website, shopping cart software can be built around them. All you need is something to sell and good online shopping cart software. Here is a short step by step primer on adding a shopping cart to an existing site:
Setup the cart – This is where the shopping cart script or service is configured for your website. This will be information like site name, pages and even payment processors. Some online shopping cart providers will also payment processors built in. Alternatively there are several third party processors you can sign up with that you will need to enter the details to the website shopping cart.
Add products – This will include adding all the products and the categories. Generally you add the categories first and then add all the products. If the products have different sizes, colors etc then those variables will need to be entered. This is usually the most painful part of the process. Sometimes adding hundreds of products with several permutations and options is best done by importing some form of csv file.
Design – This is the fun part. Here you can play with the different layouts for the category and/or product pages. The best practice is to find something simple and then set a baseline for sales and clicks. Then tweak certain categories to see if the changes affect the clicks and sales.