[SabrinaDent]

I would also add that an off the shelf cart system will come with a fully fledge back end. This is nice because you can make use of reusable product options, discounting, automated email receipts, etc. But it's even nicer because you can get admin reporting like month to month sales figures, daily totals, most popular product rankings, and running discount totals. Sure you can track this in Excel but the point is that you don't have to. The ability to update, adjust and refund orders from an interface integrated with the store is nice, too.

Absolutely you can roll your own, but unless you have a really defined and limited product range it's just likely your needs will scale more quickly than your code, is all.