The only postal services in the UK that are VAT free are those under the universal service (ie 1st and 2nd Class).

Tracked 48, Tracked 24 etc will all have a VAT element. As ebay are charging for the postage - that is why there is a VAT element - same as with the Buyer Protection Fee - this will also have a VAT element.

why are they charging the buyer, if a seller can claim the VAT back

They have to - as it is ebay charging the buyer in the event of BPF and Simple Delivery - not the Private Seller.

The private seller in any event, could not claim the VAT back, as they aren't (or shouldn't be) a business.