It's actually efficient to do it this way. The price may be higher b/c buyers are willing to pay a premium to buy coins off-exchange.
Also interesting to note that in previous auctions it looks like USMS only conveyed a "bill of sale" for the bitcoins. We don't know what arrangements were made for conveying the bitcoins themselves. I would speculate that they were transferred to a wallet designated by the buyer, but it's possible they were not.
It's only 2,800 coins. Why would anyone bother paying above market for that block? I would assume, if I went to an exchange and purchased 3,000 coins, the exchange price wouldn't go up much. But maybe I'm wrong on that, and BTC is thinly traded? I haven't been following it lately.
One reason to pay over market value, is once you buy these coins from the US Government, the sale legitimizes your ownership of it.
That is, whatever happens in the future between Bitcoins and the Gov/Fed/Treasury, your lot might have a standing in a court of law that other lots might not.
This sale basically cleans these coins from all previous illegitimate and/or illegal transactions that they went through - whether some of these where stolen, used to buy drugs, not reported or accounted for correctly with the IRS, etc.
In a way you are getting a government issued certificate of legitimacy. And you will not have to prove anything - when others will have to prove how they acquired theirs.
I think I've seen something around ~11k BTC volume per day on coinable exchange, so you are somewhat right, especially if you spread your trades over a period of a few days.
Also interesting to note that in previous auctions it looks like USMS only conveyed a "bill of sale" for the bitcoins. We don't know what arrangements were made for conveying the bitcoins themselves. I would speculate that they were transferred to a wallet designated by the buyer, but it's possible they were not.