> they could default to only allowing apps to be sold in the country of the vendor, then expand from that to countries with an extradition treaty with the vendor's country.

You don't even need an extradition treaty. Just require some collateral for selling outside your own country. Then people selling within their own country (i.e. probably most people) don't have to post anything but are automatically in the same jurisdiction as the victim. Whereas scammers from other countries forfeit their collateral, the amount of which is set based on the amount of scams coming out of their country.