In all reality, there are likely some unethical brokers around, just as there are some unethical vets around. Unfortunatly, in any profession, if there is an opportunity to make money, some unethical person will be attracted to either the profession or to shortcuts in the profession. The regulations are there to help prevent such inappropriate and illegal actions such as theft, but it isn't perfect. The only way to completly prevent that at this time are purpose bred colonies. There are issues with those, as well (purpose bred colonies tend to have similar genetics and structures, limiting naturally occuring variation and exposure to breed differences, and there are ethical questions surrounding the breeding and keeping of purpose bred animals.)
Unfortunatly, not all rescue groups are ethical. Not all county shelters are ethical. In the past year, I have become aware of animals stolen by rescue groups (happens more than you might thing) and of county shelters that own scanners, but can't be bothered to use them, despite state law requireing the public shelters to check for ID including tattoo, microchip, and ear notching.