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Will it be a case of 1st tier buys 2nd tiers? or will 1st tiers just go for buys based on quality and not size, reach etc. Maybe the 1st tiers just aren't interested in buying other directories, so if that's true, then what will happen to all the others. Will the decent ones be 'saved' from directory heaven..
Most of the directories that are of useful quality usually are not going to end up for sale.
I do see there as being value in running a number of vertical directory styled sites, although as recently noted by RCJordan, many directory scripts are "too rigid in structure to keep up with a publisher's needs"