Most of the people that use FB groups and CL are professional unemployed chicken peddlers. They stay on there and swoop every post within five minutes. You post a rooster, and they just lost theirs and need one, you post needing a rooster, and they've got one for you. When they get more birds than they have room and feed for , and run out of pet buyers, they always have a meat buyer to fall back on. Have watched it happen on such groups, having been facebook friends with people I met at poultry swaps. They will contact you within minutes, their rooster just died helping a little old lady cross a street, and they need one bad, willing to meet. That bird will be in a cage with some others at a swap on Saturday morning, and if nobody picks him, the cage will be emptied at the meat buyer's truck.
You have no reasonable expectation that the bird will not be eaten unless you keep him yourself. You have several options. You could choose the rooster, shunning the rest of society, possibly moving to a forest and living in seclusion. You could take chances on CL or FB, knowing full well that his fate is beyond your control at that point. You could provide him with a humane death and consume his flesh yourself, so that his life was not wasted. You could also refrain from "rescuing" animals that are commonly considered food animals until you have infrastructure in place to provide care and housing for those animals for the entirety of their lives.