I learnd to make sure when I sell a pin to tell buyers I give no warrantees. People can inspect it and take it or leave it.
When there's a problem then may contact me and I will try to help but am not sure I will be able to solve everything.
I don't know what they complain about in your case and what game it was, but I'd just tell something like - I'm not a dealer and you bought a used machine, if you wanted a brand new one you should've bought a new one from Stern.