I'd rather be bad-mouthed by some stranger on craigslist than live to regret getting rid of the rooster that I have become attached to. Especially if he is that nice and friendly. Nice roosters can be hard to come by! LOL
I say you should keep him. Tell her the truth though. All you have to do is say you changed your mind and decided not to sell. No need to go into details. She may be very understanding. And, if not, so what. Craigslist is pretty anoymous. If you contacted her offline, then it's not like you advertised and then took back your ad. And, even then, so what? No money changed hands. But, let her know ASAP so she doesn't continue to count on getting your roo.
Just my opinion, of course.