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Is there an easy and effective way to prevent duplicate listings?
Yes! In fact there are 2 ways to do this.
1. UUIDThe UUID parameter is a unique identifier for the eBay Item Listing you are sending to eBay. The UUID parameter is essentially a 32 character GUID (Globally Unique Identifier) with the dashes removed. GUIDs consist of hexadecimal characters only. Most algorithms for creating a GUID use some variation of an Ethernet card MAC address, IP address, system time in milliseconds, random numbers, and MD5 hashes.
Using this parameter in your AddItem, and RelistItem calls will greatly reduce your chances of duplicate listings.
Here is the doc link : http://www.developer.ebay.com/DevZone/XML/docs/Reference/eBay/AddItem.html#Request.Item.UUID
For more information about UUID, please refer to this KB .
2. Management by SKU
If your listing Fixed prices items, another option to prevent duplicate listings is to list your items with InventoryTrackingMethod as SKU. If you choose SKU as your tracking preference for a listing, the value in Item.SKU must be unique across your active listings. You cannot create new listings with the same SKU value while the listing is active.
Note: If your application is a multi-threaded application, please ensure that listing requests for the same SKU are not sent in quick successsion. This could cause a race condition on our servers and result in duplicate listings, despite using the InventoryTrackingMethod as SKU.
A SKU (stock keeping unit) is an identifier defined by a seller. Some sellers use SKUs to track complex flows of products and information on the client side. A seller can specify a SKU when listing an item with AddItem and related calls. eBay preserves the SKU on the item, enabling you to obtain it before and after a transaction is created. (SKU is recommended as an alternative to ApplicationData.)
When you track by SKU, it means you can pass your SKU instead of the eBay item ID in other calls that support SKU as an input field. For more information about Management by SKU, please see Management by SKU