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Kevin CummingsBarry is onto something here. In our answers above most of us have been guilty of breaking the cardinal rule of time travel - do not change anything!
If we took all the cards back then that are valuable today, we would seriously negatively warp the underlying fabric of the fragile time/space continuum to the point that John Dreker would be the richest man on earth (since he has cornered the market on N172 Larry Corcoran cards) followed closely by Tom Boblitt (still having several cases of 1987 Topps in his garage). 