"So why don't HMRC actually write to us to ask for our National Insurance numbers?" - As has already been mentioned HMRC already have your National Insurance Number. So many people now receive money via digital platforms now it needs to be identified due to the disparity between the calculations of money within the economy and the taxes paid.
The aim is to identify not only where the missing money is going but whether it is being used or obtained from criminal activities. Tax evasion is just one of the areas identified; along with people trafficking, drugs income, corruption involving public bodies and officials, etc., etc. The aim is also to identify the cross-border transfer of money involved in such activities by digital means.
HMRC, with the software they now possess, can quickly identify whether a persons income and lifestyle matches that being obtained through PAYE, benefits, savings, etc. If there is a disparity it may flag an alert for investigation.
With regard to digital sales platforms it will allow sales income to be included in any such assessment and can be also matched against a seller's purchase history. You have nothing to worry about unless you are trading under the radar; and even then only if you are receiving money above the £1,000 trading allowance. HMRC's trading definitions are set out on their website under 'Badges of Trade'. These do not involve the selling of personal possessions unless you are lucky enough to have something that falls within the remit of Capital Gains Tax. If you are concerned or worried just check out the HMRC website; after which if you are still worried speak to an accountant rather than advice on these boards; some of which is good but in many cases ambiguous, or just plain wrong.
