We just finished a pass making all pets that are caught via pet battles in the world non-tradeable. This means these pets cannot be put in cages for trading or posting on the AH. We felt that the option to buy these pets on the AH would take away from the exploration/collection gameplay of the system.
This could always change in a future patch, but this is how it will go out with MoP. You should see the change in our next build.
I see I've not been the only one considering the consequences of making Wild Pets tradeable on the AH.
This is a significant change to the game plan. It will mean that the value of current 'live' pets is likely to sharply rise as a result: what is not yet clear is whether you'll be able to sell tradeable pets above level one (which should be the starting level for all existing pets in your inventory come Pandaria) It also means that all of us who have been collecting their stockpiles of cute over the last few months won't have to deal with an invasion of competitors once the Pet Battle gates are opened.
I, for one, am amazed Blizzard didn't work out this was going to happen. Just goes to show, sometimes even the Big Guys don't think of everything :P
Cheers to @TheMcfluffy on Twitter for this heads up :D