Dealer's Only ONE Single Fit-all Standard for "Legal Collecting"?

Bab Dadge (Artemis Gallery) sells some of his artefacts with the guarantee:

All items legal to buy/sell under U.S. Statute covering cultural patrimony Code 2600, CHAPTER 14, and are guaranteed to be as described or your money back.

Except the ones where he does not.

This is what he means. But of course there is not just one law covering the sale of cultural property, is there? Note that he does not say that he has ascertained (and can verify) that their sale by him has not involved infringement of the laws of the countries from which they came or passed through. Why would that be, when that is the nature of antiquity trafficking? Also, does North America not produce ancient art? Or is there some problem in brushing off the legal issues with items from the territory of the USA?