Find the answer to your question
Why am I getting "Input data for tag <Item.ListingType> is invalid or missing. Please check API documentation" error message in the AddItem call although I have set the ListingType to Auction?
If you want to indicate an auction format for your listing, you need to set the ListingType to either Chinese for single quantity items or Dutch for multi-quantity items. Auction is not a valid value for AddItem call. It is meant only to be used for GetMyeBaySelling. You can find more information on the ListingType values from the Documentation.