Why you are selling the bird is a personal thing. As we would all like of course to see you keep you bird.
You can go about selling him in 2 ways...
1. Hand him over to the first bloke with enough cash to make you happy... or
2. Go about it similar to a rescue. You have internet go get some copies of the adoption forms from a rescue a good one is www.maars.org.
Ask the same questions they do. Go with gut feelings and the person who sold their bird to get a bigger bird is a real no-no. Avoid parents buying for kids. Have them handle the bird if possible. I ALWAYS ALWAYS suggest if someone is truly interested in your bird and they are bird knowledgable they will not only UNDERSTAND but respect the fact when you tell them to go home and think it over and come back a second time!
That hopefully will do one of two things, scare off the compulsion buyer and bring you a bird friendly new owner! Good Luck BTW where are you located?