There are some tuners with good AM performance, but did the buyer make clear to you that he was buying the tuner to listen to AM? Like Onhwy61 said, the buyer has no one but himself to blame if he did not. I, like you, would presume a person was buying a tuner for FM listening.
AM tuner performance?
I sold a Marantz 150 AM/FM tuner to a buyer who's now disgruntled that the AM performance is so dismal, and that I didn't disclose this up-front. My response is that I've never known an AM/FM tuner to pull any AM signal whatsoever in an urban setting, that many manufacturers show they concur by making FM-only boxes, and that I'd just assumed he wasn't expecting to use the Marantz to hear AM. Who's right here?