Just one clarification in all these speculations :
A private seller can not subtract the VAT from the goods he's selling as there's no VAT on second hand stuff. The VAT is collected by the last VAT registered retailer/seller before a private buyer and then payed to the state collecting organisation. Concerning import/export, there's a local VAT applicable on any goods coming from a foreign country being it new (all goods) or second hand (on all goods above the incoming country maximum untaxed price). Why ? Because VAT has not been collected yet by the incoming state...
For those sceptical, I have (amongst others, hélas) an European Diploma in Enterprises Management (91% score for those who wonder) and know very well those regulations, but for now all I'm reading are just speculations as the official 1500 pages Brexit deal has not been signed yet, nor read by anyone here...