If you want to turn your existing e-commerce site into a mobile marketplace with the least amount of hassle, Convertixo is a good choice. It lets you convert your site into native mobile apps with synchronized functionality and updates. It offers push notifications, social login integration through Facebook, Google and other services, and access to a large user base through Android and iOS apps. Pricing ranges from a free plan with an Android debug APK to $79 for a premium plan with both Android and iOS apps and source code.
Another good option is QuickBlox, which offers APIs and SDKs for chat and calling integration into your mobile and web apps. It includes push notifications, but also more-sophisticated features like voice and video calling, video conferencing and encrypted file transfer. QuickBlox supports multiple platforms and has a strong security focus, so it's good for developers who need something adaptable and large scale.