goodroi - 12:13 pm on Aug 6, 2013 (gmt 0)
If by link buying you are referring to spending money on a process that will lead to a backlink, there are many websites that you can legitimately do this without facing a Google penalty. Google has no problem when you spend money on a quality & relevant directory submission that requires you to pay for a legitimate editorial review. Or you can pay for a legitimate press release service to announce a new product/service in the hopes that newspaper writers will pick up your story and link back to you.
What Google does not like is blatant link manipulation. When you buy a directory link from a directory that doesn't even cover your industry - Google does not like that. Or you buy text links from blogs that sell off-topic links to anyone - again not Google safe. Or you pay for a press release just to get links from the scrapers that auto-repost that press release rss feed. Those links are not going to significantly help you and can potentially endanger your Google rankings.
There are ways you can buy links that Google would not like and get away with it at least in the short term. From my point of view that is often not the wisest long term business move.
Google tends not to forget about bad links and websites caught taking shortcuts. I also think that Google is moving towards usage signals which off-topic manipulated links will not deliver. I tend to invest money in processes that will deliver long term backlinks that send me direct traffic. The backlink juice I get for my Google rankings is a secondary benefit to me. The more I make my site less reliant on Google traffic by developing and keeping direct traffic, the more Google tends to reward me with higher rankings.