The official MercadoLibre .NET SDK by MercadoLibre calls the MercadoLibre RESTful API which provides tools to create an online store. This SDK is used to handle the OAuth WebServer flow, and there is no JSON parsing.
In Chicago today eBay will kick-off of their annual eBay Developer's Conference by announcing Project Echo, a way for outside developers to integrate their apps "directly onto the world's largest ecommerce site".
The PayChannel API makes it possible to incorporate its credit card processing gateway into your software platform. Public information on the API is not available, though it does provide 8 payment features.