Things that are ordinary in one country and banned in the next
Rules diverge faster than products do. Something routine in one jurisdiction can be prohibited a border away, and the gap between them is a real market: supplements are the obvious example and far from the only one. The interesting question is which gaps are stable rather than about to close.
The open question. Where is the gap wide enough to be a business and narrow enough to be defensible?
What would kill it
The reputational cost lands before the revenue does, especially for anyone whose other work depends on being trusted.