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Because the ivory would still feed the global desire for ivory.
And it should be obvious that if a loophole like that it would be left open in places with high corruption like your typical African...

Of course there are. But there are also tons of illegal ivory.
And while ivory can be dated, ebay has no way of knowing whether an 1870s Joseph Rodgers was rescales with illegal ivory or not.
And...

The problem i sprobably that elephant ivory pre-1970 looks exactly like post-1970 ivory.
And 'everyone's' ivory is of course pre-1970. We keep getting more and more pre-1970s ivory every year :)