You cannot , there is no email facility for eBay, it's either a Live Chat or sometimes a call back system is available.
Why cannot you contact your seller? If they are still on eBay just open the relevant case you need in the Resolution Centre if you are looking for a refund for an item.
https://resolutioncentre.ebay.co.uk
Or, have eBay removed the seller from their site? You then need to claim any refund through a case opened with Paypal.
