I think the problem really with this new system is that unless the seller registers for the UK and EU systems or uses a third party like ebay/etsy/amazon, then the handling charge due from the customer in addition to the taxation on small items will be a substantial percentage addition to the overall cost.
In the UK there would for instance be a charge of 20% tax on the value of the item plus shipping but then an additional charge of currently around
16 to 17 dollars for handling. An item which was sold for 20 dollars and cost ten dollars to post would then have additional charges of around 23 dollars due before delivery.
I think the system is pretty much the same in EU countries although in many cases both the taxation percentage and handling charges are higher than the UK and that the removal of the 22 euro exemption is particularly an issue with this kind of example.
I understand that sellers will be concerned that customers who don't understand the charges will refuse to pay and then complain to the seller.