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Use the ReviseFixedPriceItem call to change the properties of an active fixed-price listing.
Here is a C# ReviseFixedPriceItem sample that demonstrates how to delete the item's sub-title. This sample has been written using the .NET SDK v817.
Below is the code. Corresponding SOAP request is attached with this article. Please note, this modifies the item with the following SKU.
You will need to first make and AddFixedPriceItem call to create an item with this SKU.