Hi Kari,
In accordance with the terms of service associated with purchasing a custom Auctiva store domain through Auctiva, your custom domain remains managed by Auctiva and can on;y be used to point to your Auctiva store.
Since you would like to use that domain to point to your eBay store, you have the option of purchasing the domain outright from Auctiva for a flat fee of $19.95, which would allow you to manage its DNS settings however you'd like. If you are interested in doing so, please file a support request using the appropriate link on our help page: http://www.auctiva.com/free-auction-software-help/help.aspx
-Mike