My friend has had exactly the same problem giving away a garden shed. First she advertised it on her local Facebook page and immediately had someone saying they wanted it and would dm her to arrange collection. 2 other people asked to be kept informed if, for any reason, the first person dropped out. None of them made contact again, despite several prompts and reminders. So she advertised it on Freecycle and again got several positive replies, one of which confirmed a date and time for collection. Never turned up and didn't respond to phone calls or messages. After 3 weeks had elapsed, she had the shed taken away and disposed of. A few of days later, and a fortnight after he was supposed to have arrived, a man with a pick up truck turned up to collect the shed and was furious with her that it had gone!