A couple of years ago, the USFWS put commons and browns on the endangered species list due to similarity of appearance to Yacare and also because they are considered an invasive species now that they have taken up residence in south Florida. This effectively shutdown the imports of these species becuase of the paperwork required and the difficulty getting approvals (essentially not possible for the pet trade).
The Federal laws only pertain to what comes into the country and what is transported from state to state - and is in effect regardless of state laws. Any animals that were already in the country and remain in thier "home" state, or animals that have been captive bred and are sold within the same state are not controlled. Any animals of these species that are sold, traded, bartered etc. across state lines have to have the proper paperwork or it's a violation of the Lacey Act (interstate trade and transportation of endangered species) and getting cause would result in serious trouble. All this can be found at the USFWS website in the laws section. You can, however, transport the animals if you are NOT selling, trading, bartering, or in any other way receiving anything of value for the animal. Keeping proper paperwork is essential. While it would be up to the Fed to prove your guilt, it could be a long, expensive road for a private individual to have to defend themselves.
People often refer to the fact that they see ads in the classifieds selling them. These people are either ignorant of the laws or are banking on the fact the the USFWS is understaffed and therefor will not likely catch them. I think it boils down to that old Clint Eastwood line "Do you feel lucky...?"
I hope this is now cleared up for you.
Bill