OsCommerce and Zen Cart are both first generation open source ecommerce projects.
If you open a store using either of them, be prepared to switch to some other platform in the future because their respective user bases are dwindling. They are a bit old school these days.
VirtueMart is not a pureblood ecommerce platform. Its an add-on for Joomla, just as Ubercart is an add on for Drupal. If I was looking for something to fill the need of simply being an ecommerce solution I probably wouldn't look at either (although Drupal Commerce for D7 might be a worthy contender when it matures a little more).
As for WP ecommerce, well I think it has gotten dramatically better over the last couple of years. I don't buy into the argument that it is not a "serious" solution. It depends how extensive your needs are.
But if you are looking for a "best of breed" solution, IMHO it comes down to a choice between Magento and PrestaShop.
Magento Community Edition is more suitable for medium to enterprise level companies because it is expensive to maintain (you need specialized server set ups and you WILL need to pay for expensive developer fees at some point).
PrestaShop is a better solution for small to medium size enterprises. It is open source but is being driven by a solid company with offices in Paris and Miami. They won the Packt Press "Best Open Source Ecommerce" prize last year.
If you absolutely MUST take credit cards on line however, forget all of the above.
You'll need to sign up for a monthly hosting plan with the likes of Shopify, Magento Go or Big Commerce. Personally, I like the looks of a smaller outfit, Pinnacle Cart - they have better multilingual/multicurrency ecommerce support than the other three.
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Last edited by TWD; 09-16-2011 at 09:55 AM..