This is very bad selling and undoubtedly disappointing for you.
Refuse the request for cancellation and say you do wish for the item to be posted.
No one can force a seller to sell an item, but you can if you wish contact eBay Customer Services, explain the reason quoted for the cancellation is untrue , ( unless they cancelled for out of stock, this will give them a defect on their account), you did not authorise the cancellation, and you were disappointed not to receive your item. CS will review the case and may place that damaging defect on their account.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to 10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends.
I recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
You can leave appropriate feedback too. Don't mention cancelation and refunds, the feedback could be removed, something like..... Disappointed the seller did not complete the transaction.
@dorr0