Hi discount-eyewear,
That API error message you are referring to is actually a non-specific error message we display in your account when the error returned by eBay is not recognized by our system, which is why it says Please contact Auctiva support for more information on why your listing has failed to post.
Please contact our customer support team using the appropriate link on our help page, http://www.auctiva.com/free-auction-software-help/help.aspx , and we'll be happy to look up the exact error returned by eBay for you.
-Mike