No need to take it apart. Water, dish soap, sponge, rag will work. Cleaners like Simple Green will too. Don't use anything too harsh that might hurt the decals. Don't scrub with anything abrasive like a green-backed sponge scrubber. Don't aim hose stream right at bearings. Get cable housings, bar tape, hubs, chain etc all nice and clean. Then you can oil it - or maybe not, let the buyer deal with that, but at least spray some WD40 on the chain to prevent rapid rusting. Make sure everything works - brakes, shifts, tires full, etc - so that it makes a good impression on the test ride. Get it sold before winter, when bikes are worth less.