Most extensions have guidelines on ownership restrictions. However, because registries tend to be liberal about the interpretation of such rules, since strict implementation would mean reduced revenue.
Technically, you have to follow .us nexus requirements. In current practice, people (who might not qualify with strict implementation of the rules) simply claim that they are doing "significant business within the US", which if you are buying and selling domains with Americans might be loosely interpreted to make you eligible.
The situation could change of course, but as was shown with .org trying to implement their not-for-profit rule, once people get used to a certain way of doing things, it becomes near-impossible to change.