The problem with buying pets on a whim is twofold.

1. The animal gets abandoned. They never pick up the necessary skills to survive on their own in the wild, they die miserably by starvation or exposure.
2. The breeders see the high demand (especially after something like ninja turtles or finding nemo) and breed animals excessively, when demand dies off thousands of excess pets that no longer sell are abandoned or euthanized.