This shouldn't be happening on ebay.
It only happened because you allowed yourself to be talked into breaking the rules and paying the seller privately outside of eBay. By doing this you have left yourself with no buyer protection and enabled the seller to evade paying any fees due to eBay. That's why eBay insists that all items should be bought and paid for through eBay's checkout. If you speak with eBay, this is what they will point out to you.
Unfortunately by paying by bank transfer you have left yourself with little chance of getting your money back.
You can report the seller for soliciting payment outside of eBay: https://www.ebay.co.uk/help/buying/resolving-issues-sellers/report-issue-seller?
As you have been a victim of online fraud you can also report your experience to Action Fraud. Your seller looks to be a small, private seller rather than a front for organised crime. If so, and they even gave you their own bank account to pay, the police might even be able to successfully investigate your case.
