I'm not saying I'm all for this, just tossing it out as a concept. A lot of times people have multiple things to sell at once, like ProjectD said. Would it at all make sense to have subsections of classifieds?
Keyboards - complete boards & directly related (wrist rests, cases, custom services)
Caps - singletons, sets, storage cases, I can't think of anything else, but maybe there is...
Other (the off-topic of classifieds) - shoes, microwave ovens, mice, external drives, sexy santa suits...
the 3DS - that's other, completely unrelated. A FS thread titled....
[FS] 2 keyboards, keycap sets, novelty keys, and 3DS
...is kind of crappy. I would much rather see 3 subjects in targeted areas...
[FS] 1988 IBM Model M, Filco Majestouch 2 TKL, ErgoDox Acrylic Case (Unused)
[FS] (2x) R4 Spherical Set, Various Novelty Caps, purposely-melted candy corn Clack
[FS] Nintendo 3DS + Zelda
That said, I'm not much of a shopper, I don't go browsing the classifieds looking for anything and everything, I'm usually targeting something, so having things split out is easier for me.
The other idea I had would be to offer tagging of posts (from predetermined lists) so the classifieds area could have filters applied to, but that's a lot more work (so somebody better get started tomorrow).