(1) They'd have to pay at least some premium for a big block, anyways;
(2) Wealth creates more wealth and given spread and market price differentials this is more true for arbitrageurs and high or pseudo-high frequency traders today (I say pseudo because this would probably hold with such a large pool even without speed advantages and dark pools); AND
(3) Most importantly, there is a value multiplier effect for every dollar over market that these sell for. For instance, if these sell for $610 people will be ecstatic and the market price will probably jump past $620 because everybody will be ecstatic for dodging the bullet. $620 probably brings us close to the interim high of $666. You probably get diminishing marginal returns on this idea past $620 or $625 but I am just pulling a number out of my backside (admittedly).
In short, they'd be doing themselves a favor by accounting for the market's response to their winning bid -- at least I think. Then again, people have told me today that my posts are bizarre so who knows.
You are totally right. Although I do not think the bidders were probably savvy enough to realize this or to take a bid on that chance alone.
I do however think there were some bidders who:
A) wanted to win because they are competitive
B) wanted government "sanctioned" coins
C) where willing to pay for convenience and transparency