I understand that you don't need a license to breed, but do you need a license to sell?

My breeding projects will be very small-scale when the time comes, certainly not a business, so I understand that I don't need a license for that and I don't need to bring it to the attention of the IRS unless my income exceeds a certain amount.

One person has told me that he has a pet dealer's license. At what point does a person need that? I've heard that licensing differs depending on your location, but I'm not entirely sure where to go or who to ask to find out. So, who here as any kind of license? What did you have to do to get it?