Has he refunded you? If not, after 7 days from the sale date you will be able to open an item not received case under eBay's money back guarantee. This assumes that you paid through eBay's checkout and by a method that is eligible for the MBG, see: https://pages.ebay.co.uk/ebay-money-back-guarantee/
You can of course also leave appropriate feedback.
Your seller may try to cancel the sale to avoid being penalised by eBay. If they falsely claim that you agreed to the cancelaltion this can be reported to eBay,
