If you are talking about MP3s you can definitely be liable if anyone trades or sells copyrighted music. (Remember Napster

)
If your discussing actual CDs/tapes/records then it shouldn't be a problem provided:
1) You make it very clear that everything sold must be OEM recordings and NOT copies of recordings.
2) Provide a disclaimer that states, "We do not warrant or make any representations regarding the use or the results of the Classifieds in terms of each listing's correctness, accuracy, timeliness, reliability, availability, or otherwise. The author of the classified listing shall be solely responsible for the accuracy of the information provided."