The chief criteria for judging an ecommerce shopping cart are number of credit card processors and shipping services it supports, and number of people that support *it*.
Why? Because credit card processors and shipping services mutate all time. Your cart will require updating. Which service works today may go out of business tomorrow, and leave you with orders piling up.
Other important criteria are how easy it is to set up, and add products, and how easy it is for customer to use.
When you set up your shop test it using a wide variety of oldest and buggiest browsers you can find. If your web store works under them you're home and dry.
Here are cheapest, simplest, and most effective carts I've found:
1. Oscommerce (free) - http://www.oscommerce.com
A very good, full-featured, cart. Uses Php and MySQL. Not easy to set up for a 'newbie'. Cookies are used to track order. If you have PhpMyAdmin installed in your web account, it's easier. Requires a customer to register before they can make a purchase. Supports a wide range of credit-card processors and shipping services.
Bad point: Technical support is limited to Oscommerce forums, which are not helpful to newbies.
Also, it may be a while before an update is available to a payment module. These are done by unpaid enthusiasts.
Good point: Oscommerce is supported by thousands of unpaid enthusiasts; this means updates do eventually arrive, and it's less likely to go out of business, unlike a commercial cart.
2. X-Cart (commercial) - http://www.x-cart.com
Similar to Oscommerce. Requires a customer to register before they can make a purchase. Lots of features and add-ons. Supports a wide range of credit-card processors and shipping services. Has an affiliate program add-on, and lets others sell products though your cart.
3. Dansie Cart (commercial) - http://www.dansie.net
A well specified cart. Supports a wide range of credit-card processors and shipping services.
Bad point: Apparently Perl code is obscured, to make it harder to copy, which is annoying if you want to customise it.
3. Interchange (free) - http://www.icdevgroup.org
A version of old Akopia / Minivend carts. Complex product with lots of files and a lot of setting up to do. A complete solution, and includes option of third party credit-card real-time order processing. Encrypts orders.
4. Agora (free) - http://www.agoracart.com
A Web-Store/Commerce.cgi hybrid.
5. The Commission Cart (commercial) - http://www.siteinteractive.com
A CGI-based shopping cart which also functions as an affiliate program. Other webmasters earn commissions by signing up and linking to your site. 6. ShopFactory (commercial) - http://www.shopfactory.Com