If the previous owners left some of their music on the player, this can be a clue, especially if the music is liked by relatively small demographic groups. For instance, if FID 120 (the first non-playlist number available for tunes) is by Jefferson Airplane, FID 130 is by Justin Timberlake, and FID 140 is by Eminem, you can use that to work out that your player must earlier have been owned by a faded Bay Area hippie, a fourteen-year-old girl, and Eminem, in that order.

Um, no, actually I don't know of such a place. You could try doing a search on this BBS, but the sort of owner who puts their serial number in their signature file probably isn't the sort of owner who would ever have sold their player on in the first place.

Peter