Find the answer to your question
I am able to insert "Auction" with a Starting Price and
Buy It Now Price or just a Starting Price, that works fine by default.
However, I am unable to set a "Fixed Price" insert.
For ListingType.Chinese format listings, the allowed price types in AddItem call are:
Item.StartPrice - the original listing price
Item.BuyItNowPrice - allow a user to purchase the item at a fixed price
Item.ReservePrice -the lowest price at which the seller is willing to sell the item
For ListingType.FixedPriceItem format listings, the allowed price types in AddItem call are:
Item.StartPrice - the constant price at which a buyer may purchase the item
ListingDetails.MinimumBestOfferPrice - the minimum acceptable best offer price
When selling a fixed price item via eBay SYI flow, you are represented with the Buy It Now price tag in Pricing and duration section of the SYI page, however; the meaning of the tag is not the same compared with the API price properties or concepts. The price a seller entered in the Buy It Now price text field on Sell Your Item: Enter Pictures & Item Details page for the fixed price item is actually mapped to the Item.StartPrice property.
Document: AddItem Request