I have people that I deal with that I have gotten to know over the years - sometimes it's just a random person - but mostly people that I know. I put out a call to all of them first, if no one needs a rooster right then, I go to craigslist. I'm always selling chicks, not full grown...
I have been selling my roosters for so long, I generally sell to the same folks that I know for breeding or butchering. Sometimes I have 5 or 6 at a time to sell.
I sell my roosters on Craigslist or donate them to 4-H kids as soon as they begin to try to crow. I can't keep roosters 😟 You could also check with your local 4-H groups and see if any of them have roosters they need to cull. When you hatch chicks, you will always have about half roosters...