Someone us threatening to sue for a defective washing machine I sold.

I bought a house last year which had two washing machines & dryers hooked up. I did not want the second set, so I took it out as soon as I moved in (I never used it) and sold it on an online marketplace for $400.

I was clear in the listing that the machine had not been tested and I did not know if it worked.

The buyer messaged me three weeks later saying it doesn't work and they want a full refund, threating to take me to court if I don't comply.

Am I at fault here and/or legally obligated to refund them? I do feel bad that it doesn't work, but I sold it cheap precisely because I did not know its condition beyond the external appearance.