Animal Fair seems to break how buying works and how it's described in the rulebook:
Then, you can buy one card, costing as much as you have or less. ... You buy a card by choosing it from the Supply, and then "gaining" it.
With Animal Fair you can now choose a card that costs more than you have. Which of the following is the case?
1) The rulebook is inaccurate.
2) The rule for buying is now changed.
3) Animal Fair has some kind of special unstated rule attached which means you're allowed to choose it when buying even if you can't afford it.
In case 1 or 2, what could be the rule? "Buying means choosing any card, then trying to pay for it, then successfully buying it if you succeeded in paying for it"? Note that we can't just define "successful buying" as "leading to gaining" because of when-buy abilities. (If you could buy any card and then fail to gain it if you can't afford it, then you could buy cards you can't afford just for Goons-points.) The problem is that you can't define buying as "trying to buy". I mean, if "buying" mean choosing a card and trying to pay for it, then by definition you did already buy it, whether you could afford it or not.
I guess it would rather have to be: "In you Buy phase you can choose any card, then try to pay for it, then if you successfully paid for it, you buy the card." Wait, this rule actually makes Animal Fair not work!
When it comes to alternative 3, I'm not going to try to guess.