The most popular shopping cart platform is Magento. It’s an open-source PHP script and has a huge community of support, extensions, and themes. Out of the box, it has many features (so many features that it often overwhelms new users). It is a very robust platform and financially backed by the largest eCommerce players (eBay/Paypal). Because it is an open-source you can mod it as needed.
Thousands of themes and extensions are available for Magento and nearly every eCommerce serve has already provided integration and support for this cart.
You could literally have your website up and running in a day or 2 if you use the proper extensions and start with a theme designed for your industry.
i.e. Booking and Reservations by aheadWorks – Magento Connect
For even more see this link: Results: booking reservations – Magento Connect