Online Shop Development
Various Open Source Ecommerce Shops compete on the German market. All offer broad functionality to appeal to a wide range of customers, either build-in or through extensions. From the user's perspective, they have much in common, meaning that you have a catalog with categories and products, payment and shipping options, a content management section to edit static pages, an overview of orders and a fulfillment workflow. Important differences are to be found in the programming style, the licensing schemes and related services, and also the strategies and cultures of the software companies.
I cannot recommend any particular shop software. Only a thorough analysis of the actual requirements can yield an answer. I have worked intensively with the Magento Community Edition, and a little with Prestashop. Both are fine if you want a customized standard shop. If you want a more individualized custom shop, you may be better off with the Spree Ecommerce Platform, which is convenient for developers to implement custom solutions.
Integration/Connectors