Secondly people who think that the coins would be dumped just don't understand the sort of people who are bidding. They are investment firms etc that have no interest in just dumping on the market for a few thousand $ profit, they are after liquidity for their ventures.
There is no evidence for those claims.
The first 30'000 were snapped by Tim Draper who said he intended to "provide liquidity" for Vaurum, an enterprise he had invested in. Both on that and the next auction, SecondMarket formed syndicates of small bidders who could not afford to bid for a whole 2000 lot; and such bidders grabbed 48'000 of the 50'000 coins. The other 2000 coins were bought by Tim Draper again, this time to fuel his son's Boost incubator.
SecondMarket bid on their own this time, not for the fund but for their OTC trading desk.
"Provide (BTC) liquidity for" some market means "sell those BTC to clients of that market as soon as the bids get a little above their buying price".
OTC markets and the open markets on the exchanges are all connected by brokers-arbitragers-marketmakers. Dumping 50'000 coins on the OTC market has the same final effect as dumping on the exchanges, only that the damage will happen gradually over weeks or months, rather than all of a sudden.