As a designer, I can understand the standpoint. But, the way our contracts are worded is that all of the IP is owned by us on the grounds that it is our code. We wrote it, we have full rights to it (to use it again, to sell it, etc) while the client has no actual rights to resell our software or design. They can, however, sell off their domain, including our software/design, and the owner is responsible for the software agreement (always included in the software/design files somewhere).