I know what hes worried about. I worry about the same thing. You just dont know where that rooster is going to end up. I personally worry that by advertising in a local paper etc "free to good home" means theres a good chance the rooster could be collected by someone who would harm them. by way of Cock Fighting or even to tran their pitbullsto kill stuff. Maybe even as greyhound bait. Its a cruel cruel world out their with some very cruel people and they look just like us. 
I'm currently researching how to kill roosters. I probably wont be able to. I have done it before and I found it extremely distressing to do. I feeel like I have to wear it. I mean, its worse for the rooster right? To me, and this is ONLY my personal opinion, its just too darn dangerous to just give them away to people you dont know, and when you cant see where they are being taken.  
We can keep one rooster now. Our new neighbours are cool with it. Im still going to try a night box, see if it works. Hopefully that would keep the neighbours onside longer. But we are incubating chicks and weee may end up with a helluvalotta roosters. I struggled to give one away when I tried once. It will be impossible if I have to give away 12, 18, 24 or more. I am going to have to learn to llive with klling a few roosters. On the upside I supose we can eat them if we can grow them first.
Is there a thread on this subject. You know, methods, the best/easiest age to cull them. Is that the correct term? "Cull". I ask because I can not find very much on the subject.