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If you list an item with product data, your item has better chances of being surfaced in the search results. Here is a C# AddFixedPriceItem sample for listing an item with product identifier. Both UPC and MPN are passed in the request and eBay uses the first matching product to prefill the item listing.
This sample has been written using the .NET SDK v673
Listing with Product Data
Below is the code. Corresponding SOAP request is attached with this article.