Well, first off there's selling excess animals to the Animal Sanctuary, they don't pay much but it's a good place to get rid of animals who are Old, or unsellable, or just plain ole' unwanted for some reason. Quick cash for unwanted animals. Also there is no waiting, they are gone instantly from your pasture, just be careful as once sent you cannot get them back. I use a special pasture for sending to the A/S (Animal Sanctuary), I call it Retirement Acres, only unwanted animals are moved to this pasture, and it is the only pasture I open from the A/S page. Prevents any accidental sending of wanted animals!
There's also the Fair and Auction House, in the Fair you can sell animals for a set price which you set when you put them in.
The Auction on the other hand works like a R/L auction, sort of like EBay. You put your animals in for a starting bid and hopefully people will bid on your animal and raise the bid. This works well for really nice or rare animals as they are more in demand and the bidding can get quite high on an excellent or very rare color animal. Animals can be removed from the Fair at any time, and from the Auction House if there are no bids on them.
You also have the Shows and Races, if your animals are good enough to place you can make some money off of the Shows. Likewise the Races are a money maker if you are good enough to win, but you can also lose money if you aren't good enough or don't enter wisely. There is also organizing your own Shows and Races, and Sponsoring Races. These can make you lots of money, but can also lose you money if your not careful. Races and Shows will be discussed in more detail elsewhere in this guide.