A forum member who was into cockfighting as a youth said they would not use our roosters for cockfighting, especially our big clumsy full-sized fowl like Wyandottes. They use roosters specially bred for it. But this is just the serious cockfighters. You never know when you will get an idiot amateur that thinks it would be fun. Life doesn't come with guarantees but the chance of him being used in cockfighting are fairly low. He is much more likely to get eaten.
Once you give up custody of him you have lost control. That can be hard for some people to accept. The more conditions you have the harder it is to find someone that will take him, as you are finding out. My suggestions.
You are in the US, thanks for that info. Call your county extension officed and chat with them. Somebody there is going to be involved in 4-H and the farming community. They may have a good solution for you.
Continue as you are on Craigslist and vet as best you can. You might see if there is a local chicken Facebook group.
See if you can post a notice on the bulletin board at your feed store. Of course you have to vet the responders.
That's all I can come up with this morning. Good luck!