If the transaction is via an 'Online MarketPlace' such as eBay or Amazon, they - not the sender - add VAT to the transaction so as a customer, you pay VAT to eBay, just as you would with any other VAT rated goods from a UK supplier. eBay then pass on the VAT to HMRC.
For goods of greater value than £135, Customs Duty falls due, and whoever delivers the item collects that from the customer and adds an admin charge. (Port Office charge £8.00 for that, unless it's increased). Some transactions may come under the radar, but the days of buying something on eBay for say £150 which flies in under the radar with no VAT/Customs are over. If you buy something on eBay from China and it costs £20.00, you'll pay £4.00 VAT, and if the Chinese customs label say 'value $1.00' it's of no relevance.
This explains it, and the little chart clarifies what happens with two typical transactions:
https://www.bdo.co.uk/en-gb/insights...s-after-brexit
How 'peer to peer' transaction from say the EU between private individuals is treated, I've no idea, but items I've bought from China this year have included VAT.
Don't know what happens with Bangood or aliexpress).
Hope that helps a bit.
https://www.bdo.co.uk/en-gb/insights...s-after-brexit