Id say he has been given a vast amount of duff info, a US based seller does not have to register with HMRC, does not require a UK Vat Reg and is not responsible for paying UK VAT, anymore than a UK seller has to register and pay taxes to the US Dept of Commerce or whatever its called.
VAT & Duty is payable by the importer not the exporter, and even so i believe, but cannot currently check, that VAT is not added to amounts up to something like £30.00 and duty is not payable on amounts of around £100.00 i think it is, you may incur a handling fee from the carriage company DPD, TNT, Royal Mail etc though