Good question, and I don't have a definitive answer. But if it were me, I'd start with the state health dept. and the state agriculture dept. A lot of that info is online, you have to dig patiently to find it. Also check to see if you need a wholesale business license.
Possibly just NPIP which for our state is a yearly pullorum test on your breeding flock. There's room on the form for other tests, but they aren't required here. Private sales between parties are fine with nothing, but any public sales the seller here is required to provide the buyer with paperwork showing the birds themselves if over 3 months have tested negative or if chicks that the parent stock is negative, if the buyer asks for it. So any chicks I buy at the feed store, if I ask for a copy of the paperwork, they legally have to give me a copy. They couldn't do that if they don't get it from wherever they purchased the chicks.
For our state it's handled through the state's animal health dept. So you might check there first. For our state and from what I've heard from several other's their states as well, getting NPIP certified isn't a big deal. A tester comes out, tests your flock for whatever it is your state requires they be tested for. Some states it's free and other's it costs. If everything tests negative, you get your NPIP number and you're good to go for a year.
Call your local extension service. I used to live in Georgia. I think it costs $25.00 and they come out and just inspect where the chickens are being kept and that all is well. After that you can sell eggs as well to the public.