if you don't have the budget it will be hard. Look at all the successfull ecommerce sites. They all use either commerical ecommerce or self developed ecommerce. That ecommerce site is their bread and butter. They make sure to put in the most effort into this. Using open source would means security will a concern. Imagine what the hacker can do if they hack into your ecommerce site.

However if you have limited budget then by all means use ready made opensource. They can be effective as well if done properly.