Hi Jenny,
I just reviewed eBay's "Sell your Item" form and it does not offer the option to add Skype during the listing process either. Perhaps there is a setting you can enable in your eBay selling preferences that makes it so the Skype option is appended to all of your listings or something of that nature.
In any event, for further assistance with this matter, I recommend contacting eBay's customer support department. If you have any additional questions or concerns pertaining to your Auctiva account, please file a support request using the appropriate link on our help page:
http://www.auctiva.com/free-auction-software-help/help.aspx-Mike