Why did Widmore buy the Black Rock Journal?
Charles Widmore is an enigma. It appears that he has intimate knowledge of the island and people assume that like Ben he is an exile from the island. So if Charles Widmore has previously been on the island and used to own the island before Ben then his knowledge of the island must be pretty extensive. He provided maps to various Dharma facilities on the island to the freighter people.

Yet we see him at an auction room buying the journal from the Black Rock. What information could possibly be contained in that book that Widmore doesn't already know from being on the island?

If Ben and Widmore are in the same postion as each other, and they are both exiles that are looking to return to the Island, then I think we may have some clues as to why Widmore was at the auction.

During season 4 we were told very strangely that the island will not let you die while you have work to do. Evidence of this was provided through Michael and also through Jack's attempted suicide off the bridge. Something steps in and prevents the death from ocurring. Also in the finale we were told that all of the Oceanic 6 and dead Locke need to go back in order for any to go back.

Obviously Ben sees this as a way for him also to get back to the island.

Now what if Widmore is also trying a similar strategy? But for him it is not the survivors of Oceanic flight 815 that left the island but the survivors of the Black Rock who left the island. The journal that Widmore bidded on obviously was taken off the Black Rock many years ago and taken off the island by at least one survivor and probably more than one survivor. Maybe it's sole interest for Charles Widmore are the entries in that journal after the survivors of the Black Rock escape the island. The journal would list the names of the people who managed to leave the island. And It is probably these last few entries that interests Widmore.

Obviously these people would be long dead, you would assume, or pehaps the island is keeping them alive while they have work to do. I would imagine that if Widmore can track down these people then they would have a similar ability to return to the island like the Oceanic survivors do.

I think that Ben's hitlist might be the survivors of the Black Rock and/or their descendants. Maybe the descendants also have the same power as their forbears. Or does the island prevent people from aging too much while they have work to do?

I think that both Ben and Widmore are applying the same strategy. Why was Locke killed? I think arranged by Widmore to prevent Ben getting back to the island.

Once they get back then I guess it is control of the island that they both want. Obviously the island has even more power and purpose than we have been shown up until now.