I've dealt with this quite a bit from making pet food for a different species. There is one major, major problem with selling homemade treats. If anyone gets mad at you, they can turn you into the FDA or USDA which regulates the manufacture of food products for animals. You are required to make said treats in a USDA inspected facility, have the proper labeling (including ingredients, nutritional information, manufacturer's information, etc), and be prepared to have inspections and testing done on said treats.
You may be able to do it and you may never be turned in. Many people do this, but if you "get turned in", it's very expensive with the fines and lawyer fees.