I know in North Carolina must cattlemen, farmers, etc. run farm licence tags. I don't know if you do this in all states or not. However with a farm tag it is illegal to haul cargo for someone else and you can be fined pretty big. I'm not saying that i won't go get some nieghbor calves to take to the sale with mine, but i am saying thats your not supposed to. (do as I say, not as I do) ;-) If you are thinking about making it publicly known, you may have to look into a commercial tag. A farm tag is only good to haul your own products to and from market( thats from the dmv book)
other than that and the insurance thing, I don't see why you couldn't make a little side cash doing it.