IMO there is no such thing as a ‘bad’ link… therefore, one might infer that all links are ‘good’… but there are varying degrees of ‘good’.

Unless there is obvious and overwhelming evidence that you are buying links, I’m not sure the SEs can really penalize your site directly. But they can go after those selling links, which indirectly hurts you as a buyer because you may now not get what you paid for… and may continue to pay for the links not knowing that you are being passed Zero link equity from them.

The best links IMO are from authoritative, high PR sites that are relevent to your site, from pages on that source site where they are targeting similar or same keywords as the page to which they are linking on your site, and having the link in the actual text of the source site’s page, and the link text for the hyperlink being the keywords you’re targeting on the page on your site to which they are linking.

But even a ‘click here’ link on another site to a page on your site can only help (pass 0 or more link equity), never hurt…

If you’re buying links, know where they are coming from… what types of sites they are coming from… get references. And make sure that the growth looks natural.