It sounds like your browser may have it saved in the session that you are logged in as the wrong user.
Click the "Sign Out" link at the top of the site and the close your browser. Then when you open it up again and go to auctiva, make sure you are not signed in. If you are not, you should now be able to sign in as the Auctiva user you want to list with.
You can verify which eBay ID is associated with your Auctiva ID by going to the "My Account" tab, and hitting the "eBay Settings" link.
Let us know if you are still having problems.