Before you scream out "not an exploit", this surely can't be intended as it will affect economies drastically with the rarer leveled pets.
Battle Pets are cross-account, cross-server, cross-faction, etc.
You can cage and sell them. Level 25 pets could go for a lot.
1. Level a rare pet to 25 where you got it.
2. Go onto the realm you want to sell it. Cage and list it on the AH or advertise.
3. ????
4. PROFIT!!