I'm quite amazed where these huge sellers get their coins from.
Remember those 850.000 bitcoins "lost" on the MtGox? Somebody has the private keys of those BTC. Ask yourself what would you do if that was you. Would you hold all of them, or would you convert some to the fiat?
my guess is that if one was smart enough to steal this amount of coins he/she is also smart enough to sell them off exchange.
if it was me, selling them via a verified exchange account would be the last thing i would do.