We may have a solution that helps achieve what you are looking for. In fact, our Magento solution helps integrate with ebay. You can use nChannel to push item and inventory data to it from your POS, other eCommerce (like eBay), ERP or accounting system.
You can read more about our solution here: http://www.nchannel.com/connect-magento-2/
I hope that helps with what you need.