make sure they are humanely treated.
A meat bird isn't going to be tortured - generally, if you were to sell a meat bird (often assumed a Cornish X or a Ranger) then they will be taken home, and processed shortly. People generally use a quick method to cull, and anyone with experience knows the process goes smoothly when birds are calm and deed is quick.
If you are raising random males and calling them "meat birds" than you are at risk of selling them to those who are involved in cock fighting. However, they are the minority of buyers.
If you are thinking they are going to be sold to relatively inexperienced people, then you could provide a training class for inexperienced people so you can teach them the most humane method of treating and processing a meat bird, people generally stick with what they know and you would be giving them the know-how.
You could sell live birds at an auction - but selling price is determined by who is there and bidding.
You could sell live birds to people who feed their dogs a "raw diet". I know of a quail farm that would sell some of the extra males to people who fed a "raw diet" to their dogs.
However, selling
processed meat is another matter.
What state are you in? I'm in Ohio, and I can sell a large number of processed birds from my "doorstep" to individuals without having a bunch of oversight or paperwork, but cannot sell to businesses, restaurants, etc. But all states differ in these types of rules. You should call your extension office to see what the rules are in your state.
Selling the processed meat by the pound is generally going to net you the highest price.