Chargeback likely wouldn't apply here since that's for credit card payments, not cash on hand. It's regarding reversal of transactions and there wasn't a transaction using the bank here, so there's nothing to reverse.
From memory, so please don't take this as the final word - consumer law DOES apply. However, only for things that are not as described. If you sold it as fully working, and it clearly wasn't, then you would still be liable. From what you've said though, you were completely up front about all the issues etc., so you'd be fine.A business needs to ensure that the item isn't faulty and 'fit for purpose', but a consumer sale only needs to look at whether or not the item was as described. I know that definitley the position under the Sale of Goods Act. I *think* it's the same under the Consumer Rights Act, but haven't checked.
I disagree with the posters that he'd get laughed out of court. I've been to and argued in court on ridiculous claims and the vast majority of the time, there is a proper hearing on the matter. 3 months on doesn't matter. My guess is what it'll come down to is proving that you explained all the faults and issues to him. If you have e-mails etc. about this (not just the in-person chat, which comes down to a 'he said she said' unless there are witnesses), then I reckon you should be fine. Just a question of how much hassle that's worth etc.