This post is in response to the idea that you should be afraid of who you buy links from. 

Inbound links cannot be penalized because there is not a reliable way for Google or any other search engine to verify that the target of the inbound links is responsible for placing them.

Googlebowling doesn't exist here, because if it did then everyone would be able to get their competitors penalized out of the SERPS.

At worst, the inbound links simply carry no weight and do nothing to help a site rank.

It is very likely, however, that Google is ignoring duplicate links, e.g. multiple links to the same target from within a domain, or links that carry the same anchor text.  This could be a reason why some sites drop in the SERPS.

If getting no link juice from those types of links is considered a penalty, then it is.  But it's more of a case of the accurate value being placed on the site when they rely on those types of paid links.  Getting editorial links with diverse anchor text will never be discounted unless Google gains the key to how and where they are placed.

And even then they would only be worthless, not a penalty incurring offense. 

Links to your site can only help and never hurt you.