Verify payment with a SMS send back code. (Get the buyer's mobile number, i.e.; the country code.)
Match the buyer's IP address. (Usually in the same country.)
Due diligence.
If you cannot match the 2 above -- or other listed criterion -- then your buyer is in an unknown location.
Would the defense of substantial compliance and due diligence be sustained in tax court? Ask a tax lawyer ... A buyer of unknown location is non-taxable? No fuckin' idea ...
But the tax man cannot prove where he is resident either
