It's doubtful the police would be posting this kind of things on FB if it wasn't dodgy. Probably about £50 worth.
I had an old dear once, she was clearly not 100%. We charged her £7 for her windows, she made out a cheque for £7000. I managed to get the contact details for her daughter off her. It turned out she only lived down the road. I gave the cheque to her. I didn't want to just bin it and bugger off, the poor woman had got herself in a bit of a state when I'd tried to explain to her she'd given me too much.
It's sad that there are people out there who'd take advantage of the vulnerable.