API development for marketplace

Create two way API to communicate inventory levels with the following existing POS
Shopify
Square
WooCommerce
Lightspeed.
You need to develop an API that will call and retrieve inventory levels in Realtime, pass along a request to increment/decrement stock for specific items, as well as be called upon to increment/decrement our own stock when a sale is made on the POS platform. Items can be indexed using UPC code and another itemID for redundancy.

Our Database is Microsoft SQL, we use Visual Studio for our IDE to develop in C# ASP.NET, and GitHub for our repo.