You should be able to still leave feedback, don't mention anything about a refund or any cancellation, just very disappointed not to receive my item.
There are few reasons for a seller to cancel a paid for item, what reason did they use? If Buyer requests to Cancel, then yes you can report this to eBay say you did not agree to any cancellation and from messages sent can see you were eager to receive your item.
If you wish to contact Customer Services, I strongly recommend contacting CS 8 a.m. on a weekday when the lines open for ' live agents' as then, there's more chance of Dublin answering.
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=5190
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
@the_black_widow