Like if I fly from China to US and offer you a bridge in exchange for $20 and take the $20 and don't give you a bridge, it's a scam.
What's the difference between that and doing it online? The offer is still posed on US soil; if anything it should expose you to the legality of both countries.
Technically the US can start a war with China, which could reach the point of the US military capturing me and bringing me to the US thus ensuring I don't get away wit it. Realistically that isn't happening though. There are also trade-war options which sometimes happen in high profile cases, but often they are seen as losing more than gained.
Note that most countries will arrest me and send me to the US if presented evidence. If you used France as your example country and so I'm exposed the the legality of both countries. Russia and North Korea are most well noted as protecting their own people against crimes like this committed elsewhere, so if you can get protection from those countries for this crime it isn't a crime because nothing will happen (war of course is an option but it seems unlikely). China is a grey area - they sometimes protect their own, but often they will not, in general for this scam I'd expect they would arrest you for this scam, but not all of them.
It doesn't make it legal though; it just means you aren't arrested. The DoJ may still issue indictments [1].
[1]: https://www.google.com/search?q=doj+warrents+for+russian+hac...
I believe that's actually very rare. I mean instances in which country A extraditing to country B one of its own citizens (who isn't also a dual citizen of B). In the most common scenario, country A extradites a citizen of B back to B, or (less common) a citizen of some 3rd country C to B.
I couldn't find a single instance in which a US citizen was extradited from American soil to a foreign country, for example, even though this is permitted by the extradition treaties. (I welcome any pointers to actual instances)
Foreign countries sometimes extradite their own citizens to the US, but I believe that to be very rare. Even the case of Gary McKinnon [1] was ultimately blocked, for example.
With your bridge example different countries and jurisdictions could have different requirements for the purchase of real estate or that you even were buying real estate rather than like an NFT, toy model, etc. A scam in the US might not be considered a scam in a foreign jurisdiction and even within the US it might not be considered a scam, like if someone offers you a quit claim deed for whatever interests they have in a bridge for $20 that could be considered legal depending on what representations were made. In fact a person buying a quit claim deed for way below market value could find themselves in hot water being investigated for like elder abuse with them being seen as the one trying to pull a scam on a potentially vulnerable property owner.