Originally Posted by CRS
She has no grounds. A used item is sold as is unless other specified. If it is working when you sold it and she got to work at her place, it is a done deal.
For that matter, you could have sold her a bricked machine, she'd barely have a case - as again, unless otherwise stated, used stuff is sold AS-IS, and if she didn't test it BEFORE paying you, she has nobody to blame but herself. Hell, for all you know (and for all she could prove), she could have dropped it on the ground and broke it while getting in her car in your driveway... once it's in her hands and you have her money, it's not your problem anymore.
Fact is, the only possible way she'd have a case is if she could prove that you sold it non-working "in bad faith" - ie. that you KNEW it wasn't working at all and lied outright to her that it was... and even then, it's HER responsibility to ensure it's working BEFORE she hands you her money.