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Here is a basic sample written using the .NET SDK (v817) for listing a fixed price item to the UK site using the AddFixedPriceItem call. The sample has been written in C#.
The mandatory fields for listing an item are -
The item is set to be able to be tracked by its SKU number. Therefore a unique SKU number needs to be specified in the request. As a best practice it is recommended to list an item by specifying its Condition.
Below is the code. Corresponding SOAP request is attached with this article.