There is nothing that stops Bitstamp (deliberately or via subversion) from giving sellers infinite paper btc to trade with. They can get away with it as long as most people don't try to withdraw their coins. If they suddenly started making it hard for people to get their bitcoins back, that would be a possible indicator of this.
There is also nothing that stops Coinbase from loaning out customer btc deposits to short sellers. We'd never know if they had done so.
I really don't understand this...
I know you as a reasonable, knowledgeable poster. And, yes, what you describe is a possibility, but so is the claim that the entire 2013 rally was driven by Willy (which I don't believe either, for the record).
So why is it so hard to entertain the possibility that we're seeing the breach of the dam as a result of built-up selling pressure? We had a good rally to $680, from there on things started looking difficult, selling pressure was extremely subdued at first (for a number of reasons, but imo, mainly because the "hodl" mantra actually took hold - no pun intended), but without further fiat inflow and subsequent positive price action, it is only a matter of time before the sell-offs kick in.
Or am I being naive for considering this the most likely reason for what we're seeing now?