Hello,
I believe that at the momment, you have to pick the correct IP type (Assigned) if you want the Reseller to have his own ip. If you picked "Shared - Server" when he's created, his user account will be set to the Server's IP. Since a Reseller cannot swap his own IP, he'll have to be recreated... or he can make himself a new user for his domain. We'll see about giving Resellers more control over their own account... possibly even removing domain creation completely when the Reseller is created. (They can add their own later). We'll also give them the ability to swap their user ip, which would solve this problem.
John