Below is an excerpt from page 11 of Licensing and Registration Under the Animal Welfare Act, Guidelines for Dealers, Exhibitors, Transporters, and Researchers.
Hobby Dealers—Small-scale breeders and other dealers with gross sales under $500 per year are exempt, as long as these sales do not include wild or exotic animals, dogs, or cats. If you own no more than four breeding female pet animals, "pocket pets", and/or domesticated farm-type animals, and sell only their offspring, which were born and raised on your premises for pets or exhibition, you are exempt. ("Pocket pets" are small, nondangerous exotic or wild mammals, such as sugar gliders, hedgehogs, degus, spiny mice, prairie dogs, flying squirrels, and jerboas.) Also, you are exempt if you sell fewer than 25 dogs and/or cats each year, which were born and raised on your premises, for research, teaching, or testing purposes or to any research facility. These exemptions do not extend to anyone acting in concert with others where they maintain a total of more than four breeding females (in case of pet or exhibition sales) or more than 25 dogs and/or cats (in the case of research).