Amazon spent the past couple of years cracking down on fake product reviews, but enterprising tricksters are exploiting a fresh loophole on its site.

Many of the same vendors who sold fake positive reviews on Amazon for $5 a pop are now selling so called “list optimization” or “list maintenance” services, in which they enlist hundreds of people to vote a product review as helpful so that it moves up to the top of a product’s page.

But the votes can also be used to sabotage a competitor, voting up negative reviews of rival products and tanking demand for goods that previously had been well-received, sources told The Post.

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