Error 21683 - You did not signup to sell items on Half.com

Why am I receiving this error when trying to list an  item on Half.com?

<Errors>
   <ShortMessage>You did not signup to sell items on Half.com.</ShortMessage> 
   <LongMessage>You did not signup on Half.com to sell items.</LongMessage> 
   <ErrorCode>21683</ErrorCode> 
   <SeverityCode>Error</SeverityCode> 
  <ErrorClassification>RequestError</ErrorClassification>
</Errors>

Summary

You will receive this error if you are attempting to list an item on Half.com via the AddItem API, but are not a registered Half.com seller.

To register as a Half.com Seller:

• Follow the instructions at this URL:http://pages.half.ebay.com/help/seller/createacct_sell.html?helpsection=createacct_sell

If you didn't intend to list the item for Half.com:

• Check your AddItem Request and change the Item.ListingType to a value other than 'Half'.

• API Documentation - Half.com

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