Zentail uses your SKU on eBay for various fields. You'll notice the SKU in the "Custom Label" field on eBay. The product's SKU is also how Zentail communicates with eBay for any necessary inventory updates. 

For this reason, the SKU must be unique. This error occurs when trying to create a listing with a SKU that is already in use on eBay.
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The most common instance for this is when you've recently pushed data to eBay (likely through an update & push), but the listing has not been created on eBay yet.
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The good news is, you likely do not need to do anything. The listing will be created on eBay shortly.