Does the verified purchase label remain even after the product is returned? I think it should, in the case that you bought it, honestly reviewed it and then returned it. But that could be a huge loophole for fake reviewers to exploit.
But as you say, it’s hard to detect since obviously if someone returns the item they might want to leave a bad review.
But if all returns leave a good review? Now that’s suspicious.
Maybe amazon should show an icon if the reviewer returned the item, and force them to explain why they returned it.
But again that’s hard if they write the review before returning the item.