I see what you're saying, I imagine that the markets are just that a bunch of small businesses
selling via a larger website. In that case, It may be that the VAT is worked into their cart/checkout
system, and taken care of by Bangood/Ali-express.
When all this first came up for vendors, it seemed like there were exemptions, or that if it was below
a certain retail price, then UK Customs or the post office would collect VAT on collection/delivery.
However, with the implementation of the vendor collected system, any vendor making sales would
have to register, collect and remit the VAT for the Government. And I seem to recall that this lower
limit for vendor sales was something like £22. Which is why several US vendors I know stopped
shipping overseas. They didn't want to work for the UK, or EU government doing VAT collection.