Records: Inspect each record for deep scratches or a destroyed record sleeve. Records with small scratches will still play, but severely damaged records will just take up space and get dusty.
CD's: Check for deep scratches. A CD will play just fine with some small scratches.
DVD's: Same as the above comment.

Cassettes: Only sell pre-recorded cassette tapes. Don't sell blank tapes that someone taped a bunch of songs onto. Selling NEW blank cassettes is fine, but make sure they still have the wrapping on them.
Video Cassettes: Only sell pre-recorded video cassette tapes. Don't sell blank tapes that someone taped movies or TV shows onto. Selling NEW blank video cassettes is fine, but make sure they still have the wrapping on them.
Always organize these items so they look nice. The nice items are displayed, the more likely they are to sell.
Anything that's not up to par, toss. Unsellable A/V Audio/Video media just takes up valuable space.