Maisy:alasta: An interesting range of views here.
I personally believe that in any case where the seller has acted in good faith and has not misrepresented the goods then the burden of risk lies with the buyer. I say that on the basis that it is generally accepted that there is an element of risk associated with buying second hand goods from a private seller.
Unfortunately a Disputes Tribunal hearing could go either way because these things tend to end up with a "he said, she said" dialogue where the facts get twisted from either or both sides and the outcome depends on the particular interpretation of the adjudicator. Having said that I think it's pretty unlikely that it would end up in the Disputes Tribunal anyway because it's likely that the buyer realises that she is being petty and is just trying her luck.
Yes I had hoped that that would be the case, however I have just received an email from the buyer's husband, which makes various accusations of deceit and guilt, and demands a refund. Oh dear, looks like this is going to go to the Disputes Tribunal.
They have no legal avenue IMO.
Of course, if you think they are going to be dicks about it remember that they have your address, and I bet you don't have their address.