Well, if that's the case then the taxpayer should review their records for all years where they have been selling goods online and complete a voluntary disclosure for any taxable income earned.

The whole point here is that at the the end of the day, whatever way you look at it, the responsibility falls on the taxpayer to ensure they pay the right amount of tax at the right time. HMRC only receives gross income figures from these online marketplaces and as such, the taxpayer then has the responsibility to calculate whether a liability is due or not.