The starting point
For the majority of projects we recommend the use of cloud based eCommerce solutions. Vendors such as BigCommerce and Volusion offer merchants a powerful platform that integrates with existing legacy systems (e.g. ePOS or ERP) via well documented APIs. These Application Programming Interfaces allow us to focus on the bits that leverage your business goals rather than re-inventing the shopping cart.
We also recommend uCommerce, a powerful eCommerce platform that integrates with Umbraco - the open source .NET CMS and development framework. This provides a strong framework for projects where there is a requirement for a self-hosted solution or more demanding goals around system integration and bespoke web application development.