Build an Online Store

I'm currently running a website of classifieds ads for musicians. I want to have an ecommerce section build with opencart or prestashop (depending on what is more convenient) as a new section for the website to function as a marketplace for current and new users in the site. What i need to customise: Session between current site and this new section must be shared so current/new users can publish their services or instruments to sell to function as a marketplace. Current site is build with plain php/mysql + jquery/ajax/html/css. It has a login system - password recovery, basic user account, messaging system and a form/listing/detail for ads. I'm a frontend developer so this part will be covered by me.