Perhaps not so much a mechanical device, but take a closed source patented piece of software for example. You could be able to keep the design (source code) secret, and yet still sell it to the public.
By now we've seen this movie enough to know that if enough people care about the software, its DRM will be broken eventually. There is also a breakover point at which the more onerous the DRM is to paying users, the more users will choose not to pay and simply use the cracked version. This point will vary based on the nature of the software.
