I was going to write a post with some advice on what you should check before buying on the AH something that seems a deal, but of which you never heard before.
It would have been one of those "avoid the typical newbie's error" post... The kind of post I think is at the same time too common and not common enough.
Then I stumbled upon something I think totally clarifies one of the things I wanted to explain, so I decided to write only about it.
Most people think that looking at wowhead.com for the item they want to buy or sell is enough to understand it's value, but actually it isn't. Let's go to the comment section on the page of the [Parrot Cage (Hyacinth Macaw)], one of the rarest pets in World of Warcraft (wowhead gives him a Buyout price of 8k, but you must consider that there is people selling it without knowing what it is for just some golds... It's actual value is more in the range of 15-20k if you find the right buyer).
This guy has lost such a huge profit, and why? Just because he didn't read the date in which the first post was made, or he didn't know what it meant. This is exactly the kind of blind following of instruction that could (and will, if you're not careful), cost you a lot of money.
People fail to remember that prices changed a lot in the past years, and actually change at each new expansion, when the level cap is raised. This, by the way, is what is the "inflation" everyone is talking about speaking about Cataclysm: the most expensive mount of TBC ([Cenarion War Hippogryph]) costs 2k*, while the one in WOTLK ([Reins of the Traveler's Tundra Mammoth]) costs 20k*. Back in Vanilla, everything required less gold, and everyone had less gold...
And I'm not talking just about us AH-junkies, but everyone: before the introduction of daily quests in TBC, people had to farm hard to get their money, while now most people manage to reach a good amount of money just from these, creating a market in which vanity items are more required, since less players are struggling with mere survival and can now think about achievements (which ere also introduced in TBC) and collections.
There is also another thing that made the prices of vanity pets grow so much: the fact that they don't use bag space anymore once learned. For this, the phenomenon is even more clear in their case.
*prices before the reputation discount