If you need to build an ecommerce site for a client on a limited budget, then a shopping cart solution is a must. Sitepoint has a nice list of 18 shopping cart solutions.

Personally, I’d highly recommend checking out Magento on that list. I would also add 2 more, the hosted solution Shopify, and the PHP/MySQL solution CS-Cart. We used CS-Cart on an internal company project a few years ago and it worked quite well. As always, try out a solution before deciding which to use.