...Those 180'000 BTC were kept away from the market; and part of them, at least, must have been bought by SR customers on the open market. So Ross was obviously accumulating. I would even guess that he had been holding almost all the bitcoins that he got.
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That's important, DPR was in accumulation mode during 2013, and if now BOTH him and the US gov believe it's time to cash out, then they both expect much lower prices in the near future.
You really think the FBI talked to DPR and discussed with him whether now would be a good time to sell? They sell now because they can, the FBI are no speculators and they certainly didn't ask for DPR's permission first.