Technically, i believe (i'm not an authority on this) you need to have NPIP certification to sell from state to state. Like i said, i'm not an authority. I think this is the case. NPIP is certified by your state; they have someone come out and test for certain diseases, and you pay for a certificate. Actually it's different in every state - with the certificate - so you'll need to call your extension office or someplace like that to determine the exact procedure.
As to the NPIP, i dont' think that state to state rule is really enforced, but some buyers will ask for a copy of it.