Although the Battle Pet Trainer will only offer to teach you your race-specific companion, it is possible to add all of these pets to your collection. You can either find them in your faction’s auction house or you can create a character of the race whose pet you would like to learn. Using your new character, you can seek out any battle pet trainer in major cities. After you learn the pet, you can delete the character; the pet you learned will not be removed from your collection.
An advice for those who are trying to compete monopolizers: you cannot beat them by selling for less if they follow right steps. You should do opposite: sell for more. Because market will come down if the price goes unreasonably high. You can set a bid on item for the price it should cost and buy out price for higher than a monopolizer is selling. So, common buyer may get an item for the right price or monopolizer has to buy the item at his loss.