Ultimately, the only way to really know for sure that a chicken will get to live out its natural life is to keep it yourself for its whole life.
The next best thing is to rehome it to someone you know personally who believes as you do, that chickens are pets only, and never food. But even with this, there's still a level of risk because stuff happens. People need to move and sell off their flock. Neighbors complain about crowing. Roosters may become aggressive and too difficult to handle. Or if the person gets him for breeding purposes, as he ages he may become less fertile. In these cases, the person may need to re-sell the rooster, and it may not go to someone that you would like it to.
After that, yes, you can increase the price of the rooster and sell him to someone you don't know. It will likely reduce the risk. But in every case, you will have to find peace with some level of uncertainty about his future. Again, I think the best bet is to sell him to someone you personally know, and who believes that chickens are pets only.