Buing links vs Paying for listing in a directory

Is there a difference? When I want to be found in a directory or site that requires a payment to be listed is it considering buying a link. Also in many cases one need to pay for article to be written and posted on a blog.

Is there a line one can draw between paying for service and buying a link? And how to differentiate.

For example is this against Google policy for buying links?
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