It depends on how much buyers value their time. With the safari hat and pet treats and battle-training stones, even after the Menagerie nerf it's easy enough to level a couple of pets from 1to 25 in under an hour. If you had a level 3 garrison and the hat, would hafta figure your time was worth 4,000 G an hour to pay a 2K premium to buy a pre-maxxed pet from the AH.
Always possibility of finding a buyer that hates to do pet leveling, or doesn't have a level 100 toon yet, or is in a hurry to raise the score for his collection. That's how an awful lot of the AH seems to work, can't really complain about sellers with ridiculous prices, it's the people who buy that stuff that you gotta wonder about. But I've also noticed that the AH listings for 25's that are way higher than the 1's don't seem to move.
Personally I have extra motive for leveling my own pets, I level alts at same time. It's faster than questing.