Virtuemart has long been the preferred e-commerce solution for Joomla sites as it is feature-rich, there are a lot of extensions available and it's free. However, it is often criticised for lack of updates and difficult configuration. So if you want to sell something online, here are some alternatives to consider:
RokQuickCart - if you only have a handful of products and don't have complex shipping or tax requirements, then you might be able to get away with this extension. It is deliberately simple but this is good for both the shopper and the shop owner. It plugs in to either PayPal or Google Checkout.
redSHOP - this is quickly being seen as a replacement to Virtuemart. There is a free lite version (you need to register on the site to download) and two commercial versions.
Magento - this is a popular, stand alone, open source e-commerce application. It can work in conjunction with a Joomla site when paired with a commercial "bridge" called MageBridge.