Yahoo! Store doesn't have a ton of features -- some basic metrics and reporting, which is useful, plus templates for your checkout and website, if you choose to use Yahoo! Site Builder, as I have. Their custom site builder program is a proprietary language (not similar to HTML at all), so unless you are familiar with the language, you will require help! They do provide the option, however, of allowing you to integrate HTML pages with their system. I do believe there are less expensive alternatives, but I like the convienience of having my webhosting/website/merchant account available all in one place.
I first started my ecommerce website using CCNow, a service similar to PayPal. I believe either solution is a great, low-cost way to start selling items online.
Of course, there is always the Ebay Store solution. I haven't looked too closely into this, but it seems to be a popular choice, especially if what you have for sale would do well with an Ebay customer base!
Other ecommerce solutions rapidly appearing are the "independant" websites, such as Etsy or more retail focused sites like Stars and Infinite Darkness, formally known as Virgin Threads, and Trunkt. With Etsy you are free to create your store, while Stars and Trunkt require an application and approval.
All of these services charge fees. Don't be afraid to start small before moving to a larger, more expensive solution. The important thing is to get out there. Good luck!