Do you enjoy vanity pets? How about gold? Would you like to combine the two together? WoW Insider has a new Gold Capped article about the pet market and how to profit from selling companions. It's a pretty good read if you're just starting out and need some quick ideas to boost your income.
Personally, I've never really taken an interest in the AH (even for vanity pets), but I'm sure there are others who enjoy playing the market and may even have already struck it rich.
With all the incentives for pet collecting these days, more players are looking to delve into this once-exclusive hobby as an actual route of gameplay. Naturally there are those who are going to look for a quick and painless way to bolster their collection number, and I'm sure some wouldn't bat an eye at spending a hefty chunk of gold on a companion.
So if you're looking to get started on selling pets, check out that article. Keep in mind that AH prices will vary from server to server, but most of what the author writes is generally true.