Complete! We released a tool called Multichannel Inventory Sync to manage quantity by SKU across eBay accounts and Shopify stores.

To check it out, navigate from the left menu to Listings > Inventory Managment > Multichannel Inventory

This a direct link to instructions for this tool: https://help.3dsellers.com/en/articles/6383669-multichannel-inventory-sync-link-quantity-on-multiple-ebay-listings-channels

Thank you for requesting this feature!