A podcast indicated that it is illegal to buy \ sell a kidney in everywhere but Iran and was labelled a 'repugnant' transaction. If an American who was critically in need of the organ went to Iran to receive a kidney, does American law provide for policing / enforcing law for an act that takes place in country XYZ?
42 USC 274e(a) says
It shall be unlawful for any person to knowingly acquire, receive, or otherwise transfer any human organ for valuable consideration for use in human transplantation if the transfer affects interstate commerce. The preceding sentence does not apply with respect to human organ paired donation.
(3) The term “interstate commerce” has the meaning prescribed for it by section 321(b) of title 21
The term “interstate commerce” means (1) commerce between any State or Territory and any place outside thereof, and (2) commerce within the District of Columbia or within any other Territory not organized with a legislative body.
Travel to Iran to obtain a kidney does not constitute an instance of interstate commerce, but the law is stated more broadly, as affecting interstate commerce (the reason for inserting that clause in the law is to relate it to a constitutional power of Congress). Which brings us to Wickard v. Filburn, where the court rules that growing wheat and feeding it to your own cattle affects interstate commerce.
The interpretation of the law has not yet been tested, to determine whether a US person traveling to another country and having an organ implanted for valuable consideration "affects" interstate commerce, so that they have violated US law.