Warped or Twisted Neck
This can happen to anyone. It's not your fault. Not everyone happens to like a neck that looks like a longbow, though. Why not raise the action, put on jazz gauge strings, and sell it as a slide guitar? If this doesn't work, it doesn't mean you're stuck with the guitar. You just have to wait for someone a little less, shall we say, perceptive. (Try to find a bass player or drummer starting to play guitar.)

Dead Pickup (or assorted shorts, buzzes, etc.)
Try to get the guitar out the door without being played through an amp. (Come to think of it, try to get it out the door without being played, period. Could save a lot of explaining.) Maybe say you don't have an amp - that's why you're selling the guitar. Yeah, that's it.

Small Blemishes
Try a sticker. Sure, they look dumb, but it's cheaper & easier than fixing it. (see The Repair Guy Philosophy) Another idea: say the ding was made when _____ (someone famous) borrowed it.

Assorted Screw Holes
Remember that cheese I talked you into using for tuning? I've gotten remarkable results using cheese crumbs as a cost-effective wood putty. Pack them into the hole, soak liberally with super glue, level the surface (if necessary), touch up the color (if necessary) with nail polish, and... viola! Where was that hole again?

Big Cracks or Divots
Plan A: If they have to be fixed, use real plastic wood. Cheese just isn't stable enough to be used in large quantities (also, the guitar may begin to attract mice, super glue or no super glue). Otherwise, the procedure is essentially the same as for screw holes.
Plan B: Offer to knock a few bucks off your price. (Make sure you start with an unrealistically high price. Again, see The Repair Guy Philosophy)

Missing Parts
Say that's the way it was when you bought it. Or say you didn't like whatever it was, and took it off.