I want to develop a service based website like jobsites, travels etc with an e-commerce elements, I don't know where to begin with, what are the best free and open source resources which would be best for starting such a project.
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Maybe the best is to choose a common CMS (Content management system) and extend it with plugins. You can find a list of CMS here: Comparison of content management systems
The more common CMS are Drupal, Joomla and Wordpress. There is a lot of plugins for a lot of applications like eShop, gallery, forum, etc.
Plugins section :