I don't like to fade a well-entrenched trend unless there is a major reversal signal, and I haven't seen one in AMZN yet. I would want to see buying get really mania-like, with a couple of really large spike up days in a row, then some kind of high volume reversal day. Then I'd consider shorting it. IMO you need the shorts to really get screwed, have a day or two of panic short-covering - then that will remove potential buying support and set it up for a big drop.
Until then, I'd rather look to buy it on dips & breakouts. Why fuss about valuation when there is so much to be made from riding the trend?