As a general rule human beings make for terrible livestock. There's many drawbacks (likely to escape/protest/resist, diseases and health impacts from cannibalism, an inefficient use of resources, as well as being morally repugnant to most people, etc.) and almost no advantages (cannibalism for strictly nutritional purposes is generally a last resort when all other options have failed).

The only example I can think of that even comes close to satisfying the question would be the harvesting of victims' hair/dental gold/skin during the Holocaust. However even this doesn't totally satisfy the question, as these people were slated to die, regardless of if they could be harvested or not. In other words, resource gathering from victims was an afterthought, not the primary motivation.