Nothing wrong with it at all. Honestly, even if the ad says "not for butcher" or whatever, truth is once you purchase or take custody of the animal, it's yours to do with as you please. If I am lucky enough to sell or give a rooster away, I pretty much expect for him to be eaten. That said, I wouldn't rub it in the seller's faces, especially if there are littles around when you pick up the birds.
there was a user on her a while back who ate quite well it seemed from free CL roosters. She'd post pics of her carcasses from time to time and it looked like dinner to me.