Why the answer is not binary
A ranking change after link acquisition does not prove that one specific link caused it. Search rankings change for many reasons, and engines can choose to discount links they consider manipulative.
A better purchasing question
Ask what is actually delivered, how it can be verified, what risks you accept, and what other acquisition channels you are using.
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View BuyPBNBacklinks.net PackagesPolicy reference: Google Search spam policies. Google states that buying or selling links for ranking purposes is link spam; sponsored or advertising links should be appropriately qualified.