You might be in the right but I think you need to do right by the buyer, I expect stack would do a little more than put 12v on it. No, they won't give you their cut back.

I recently did a deal and a part had some damage that I wasn't aware of, I just bought a brand new item for £300 and replaced it, yes I am down but the deal was done and I thought it was as new but it wasn't.