Inventory and Order Management Systems

I am a Seller/Merchant on different portals like Amazon, Noon, daraz and Souq and want to keep track of inventory and products sold on these portals. These Multi-Vendor portals offer Seller Central to manage your orders and we can download Financials Account Summary, Order details in Excel format as well. I need a simple system yet with customization at all levels where i can easily add inventory of products purchased from different countries and orders received from different portals. I make 30-50 sales a day in peak times and adding them manually can be a time consuming job so looking to add all orders by uploading a excel sheet which normally we download from the portal. My hosting service is PHP and Mysql based and looking for same solution. I am looking for a developer who is good in accounting/mathematics and completely understand Multi-Vendor portals I am working with.