If you sell a bird, you really have no say in what they do with the bird. Just as no one has any say in your birds. Possible uses do include feeding families or feeding other animals. Some may be used in other illegal activities. If you suspect that, you could report it. A very few will be used in flocks.
However, most roosters are considered a financial liability, as in you feed them and do not get eggs. Extra roosters, can be very difficult to get rid of and too many roosters can wreck havoc in your flock. So if you can sell them, I would, and if you have several, I would sell as many as possible. One really only needs a very few roosters.
Roosters are a problem that every chicken keeper eventually faces. Some people choose to cook them for their own family, as they put the money into raising them. Nothing dishonorable in keeping an animal healthy, safe and secure and a quick dispatch. Others find it difficult to do this, and sell or most often give away their extra roosters.
Good luck,
Mrs K