You can report the seller using the link below.
One of the reasons for reporting is that they offered to sell you a listed item outside of Ebay.
However, as you agreed to buy outside of Ebay, I would say you are on shaky ground reporting them.
If you had followed Ebay process and paid through Ebay checkout, you would have had the protection of the Money Back Guarantee.
https://www.ebay.co.uk/help/buying/resolving-issues-sellers/report-issue-seller?id=4022
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
