To be eligible for Auto-Sync Quantity, your eBay listings must be in a fixed price, good til' canceled format, require immediate payment and accept PayPal as a form of payment.
If you choose to use this setting, quantities are synced across channels based on the listing's SKU.
Meaning, if you have a listing on eBay and Amazon that contain the same SKU, we will keep those quantities in sync automatically, whenever an edit or sale occurs.