This is one of the most important things that those "if you're so sure Apple/Gold/etc is overvalued why don't you short it?" people ignore.

Maintaining a short position is expensive. And even if you can afford it, holding it for too long can kill any upside you get when it finally does collapse. I can say that Gold is overvalued but I won't short it, and be perfectly consistent in saying that, because I admit that I have no idea how long it'll take for the bubble to pop.