I got scammed for the tiny morsel of money I mustered

I sold a bunch of things such as my laptop, and other items. He gave me fake £20 notes and I didn't notice. A cashier actually informed me when I went to pay for groceries. I had no other way to pay, and was thankful the police weren't called. I've handed in the notes to my bank, but they couldn't reimburse me. It was £280 in notes. I'm so disappointed in myself for not being more thorough. I have nothing else. Selling items was my last option. Anything will help. Thank you