I've always earned waaaaaaay more CC than I spend.

Simply by breeding the way I do and selling animals at whatever price the game suggests (because I'm too lazy to change it) or lower (if I remember to change it haha), logging in most days (even when I only have time to play for a few minutes sometimes). And because I kind of rarely need to buy new animals once I've established a few populations over my different zoos. So when I buy animals I very often buy the 10k ones without even a dent in my CC pile. xD
I very often see my own animals returned on the market because people buy them and resell them higher. (possibly offended by my cheap prices? xD I don't know...) Which... is not my goal with selling them kind-of cheap. I started doing it to give newbies a chance to grab good animals kind of cheap - and for established players to get good breeding animals cheap. Those are still my main goals but I also sell animals cheap now because I don't need the CC.

And since earning this much CC didn't really happen on purpose I'm always curious when people say they struggle with CC? (I've never done the "farming" thing either so...)
My recommendation will probably always be; try to breed good animals to sell. It has certainly worked for me.

(and I always love seeing more animals on the market because I often struggle to find ones that have good enough stats for me to want to buy them)