Cory Stockton, aka Mumper, has announced that pets caught in the wild will no longer be tradeable and that this will be the case when Mists of Pandaria launches.
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 petson 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.
In pet battles, tradeable pets can be sold on the AH. Training and caging wild pets looked to be a lucrative endeavor, since the catchable pets spawn in various qualities such as poor and rare. Many buyers would have been happy to shell out their gold for a high quality pet that they didn’t have to grind for.
As of right now on the beta, pet spawning locations tend to be heavily camped. It will be interesting to see how this plays out in cross-realm zones.