I’ve been using EBay since 2007. I’ve never ever known VAT to be added to the value of the winning bid. I have purchased items that are subject to VAT but this is always included in the value you bid.
Does the message specifically say that VAT will be added to the winning bid value? Or are they advising you that your bid, if successful, will include VAT? This would be relevant if you were purchasing the item as a VAT registered business and could therefore, claim the VAT back.
As said before, it would be useful if you could post a link to the item.