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The RelistFixedPriceItem call can be used to to relist fixed-price item that has ended. It is recommended to use this call instead of creating a new listing using the AddFixedPriceItem call to take advantage of the recent sales score associated with the listing.
Here is a C# RelistFixedPriceItem sample for re-listing an item as-is. This sample has been written using the .NET SDK v817
*Prerequisite - make sure to have added an item with the SKU below
Below is the code. Corresponding SOAP request is attached with this article.