As far as I know:
Since Ball Pythons are CITES II, you do not need any paperwork. However the airport they come in through must have avialable a check through the U.S. Fish and Wildlife. This will cost $50-$60 (total not per animal). They have to make sure the animals are okay since they are of foriegn origin and comeing into the United States.
Besides this I do not think there is anything thing else YOU will have to do, the person you are buying from should take care of the rest on his end.
For the US check, either contact your airport and get more information, if it is an International airport things will be easier. Or you can have the seller set up everything to take place as soon as they come into the country and you can simply pick up the animals and paperwork when everything is concluded.