Well, to expand on that, a lot of people don't just use the shopping cart, but all of the combined stuff like payment processing, the CMS for adding in products

Well, to expand on that, a lot of people don't just use the shopping cart, but all of the combined stuff like payment processing, the CMS for adding in products, user accounts, built-in APIs and many other features. I was involved with an ecommerce startup that built all of this from scratch to control everything legally and then sell it, and quite honestly, it is an absolutely incredible amount of work. Usually, it is easier to find software that's already been written to meet your needs unless you're trying to create a unique value proposition in some way. If you're building a standard e-commerce site, it might make sense to use some software package that's prebuilt with what you need. Either way, if you think you can do it in 15 minutes and want to build it, have fun with it and do it yourself.