What if the Ebay seller is outside the UK (registered outside the UK and selling on Ebay UK in Sterling) but the item's actually already in the UK, ie the item is sent from a UK warehouse?
From HMRC:
Goods that are in the UK at the point of sale
Importing goods to sell through online marketplaces
The overseas seller will remain liable for any import VAT and Customs Duty when the goods are first imported into the UK.
When the goods are sold to the customer, the overseas seller will be considered to have made a zero-rated supply of the goods to the online marketplace. The overseas supplier does not have to issue invoices to the online marketplace for supplies that are considered to be zero-rated.
UK VAT will be charged at the point of sale.
The online marketplace will be liable to account for the VAT on the sales made through its marketplace, unless the goods are for a business customer who gives them their UK VAT registration number.