What would the repercussions of the pool operator doing this be?  would they not tarnish their reputation? would they have to be conspiring with a number of other people all poised to double-spend at a moment's notice?

It seems like the only way something like this could be pulled off (if I understand correctly) is in an organized crime fashion.  Exchangers teaming up to contort the market for personal gain, and the gain of their contemporaries...