Craigslist transaction went south...

Be careful about blowing off possible legal action,  an attorneys has been known to take advantage of an individual's lack of legal knowledge to create serious headaches and financial consequences.    http://www.indystar.com/story/news/2016/06/06/he-sold-40-printer-then-he-sued-indiana-30000/85219140...

That case is highly unusual and in the end, the lawsuit will probably be dismissed for a failure to prove damages. In this case, the buyer threatened legal action, he didn't take any legal action (yet) and probably never will. If the buyer does bring legal action, the OP can always run scared and cave in at that time. Personally if I knew the unit worked perfectly when I sold it, I wouldn't return the buyer his money.

I agree highly unlikely,  but it did happen and notice even if it is dismissed it still cost him over $12000.  My take away point is don't just blow thing off.

As many others have said, just give him his money back and be done with it. The threat of taking legal action is just silly, as he would spend far more money fighting you in court that what his reimbursement would be, so I wouldn't stress over it too much. Just make sure that if you receive a summons to appear in court or any other documents implying a lawsuit, don't make the mistake that the guy in the above story about the printer made and respond immediately so as to avoid being the victim of a professional scam artist like Gersh Zavodnik.

so what happened?

Couple things i have thought at time before a sale of a piece of equipment..

you can take pics of serial number and I do..but what if  they swap internal boards or something?  hard to prove then.

I sold a guy a preamp on here.  2 weeks later he says it dont work and he demanded refund.  I offered some solutions to see if it was actually my pre and he ignored me. paypal automatically took the money out of my account.  even though he said he shipped it back it took another 5 additional days for him to even get it shipped out. after that I got unit and it was not only working fine but it had been banged up and scared.  I Had before and after pics.  that did not matter.  even after I discovered he had done the same thing before and contacted the other guy I still was ignored. nobody cared and the guy gave me a negative on here to add insult to injury. 

but its probably a case of user error here.  I doubt the dude is going to take you to court over $120

since you havnt heard from him maybe he chickened out, scarred to be face to face when you show it works...lol..