What probably happened was that the buyer wasn't sure how it would sound and bought it before knowing. When it arrived, he/she didn't like it and wanted a return. Like a store. another reason why I really like buyers to pick the unit up personally. That way, they would hear it work perfectly and can't complain later. Another is don't accept paypal. Paypal protects the buyer only. I never eat the paypal fee and if buyers insist on using paypal, they must add in the paypal fee. buyer is also responsible for shipping fees.
I sold a Mark Levinson ML3 amp a year or so ago. The was repaired and also modified with upgraded output transistors and worked and sounded wonderful. It was advertised as repaired and modified and very detail in the description. The buyer came to my house, and listened to the amp play for over an hour. It was great. I still regret selling this amp.
The buyer was a "dealer" which I didn't know and shipped the amp overseas to a new buyer. They proceeded to take the amp apart and then called me complaining that the amp didn't work correctly. I told him to bring it back to my house and we could connect it like before and listen. I said that if it didn't work, then there was nothing I could do because they damaged the unit. They (the new owner) could have very easily shorted the outputs and damaged the amp. hard to do on this one because it is a monster. I'm absolutely certain the new "owner" damaged the amp either in shipping overseas or when they tried to connect it and operate it.
I would not take back something that worked when I shipped it and they buyer damaged it. This is why paypal is not cool.
I understand that buyers want protection, but so do sellers. tough call here.
If the tuner works as before, then maybe give the buyer their money back and move on. but, if that tuner no longer works, then well, we have a problem. they (the buyer) broke it. More likely tho, the buyer simply didn't like the sound and considers you to be a retail store and want a return.
sorry for the problem.
enjoy the music