To resell Instances created on Amazon EC2 you need to give your customers access to their VMs through SSH keys. Customers should not see or have access to other instances setup in your Amazon EC2 account.
After customer account is created in HostBill your client can log into client portal and manage their EC2-based service. When the customer first tries to create any VM in their account they will be prompted to enter their public key from their SSH key pair. Once entered this SSH key will be used to all virtual machines created by customer under this service. With service termination HostBill takes care of removing/clearing unused SSH keys from your Amazon account.