Amazon hit with class-action suit over fake eclipse glasses

After finding out that fakes were being sold, Amazon refunded every purchaser, and told them not to use potentially-bad glasses.

How is it Amazon's fault some people were selling fake products? And, if it were their fault here, why is it not their fault every time fake products are sold?

Or is it, now, their fault always?

Because if it is always going to be on Amazon to ensure everything sold is not fake, their catalog is going to shrink dramatically very very soon, and they'll need to setup some kind of seller & product vetting process.

And just who, exactly, is going to ensure a given product is not "fake"?