When buying in the UK the seller is responsible in being confident that the buyer had a valid ‘defence’ under the VCRA, whether they are a retailer or private seller
When importing into the UK the importer (buyer) is responsible in ensuring they have a valid ‘defence’ under the VCRA
HMRC/the carrier will observe incoming packages based on their import declaration, the location it came from and how it looks or potentially x-rays. If they know it’s a RIF (from actually looking), or if they suspect it may be a RIF (Declaration, X-ray or sender) then they send out a letter asking if it’s a RIF and if you have a Defence - they may accept your documenting site membership —— that’s pretty much what the VCRA requires
Fail to convince then they may send it back or destroy it, the seller has no obligation to refund, but an international retailer will typically give a part refund less a restocking fee, a private seller will have no obligation