Nobody can say for certain what can or can not get you penalized and/or filtered. You are not likely to get banned from google for being suspected of buying links, same as selling links, however you are more than likely going to get hit with some kind of "penalty" or as I like to call it "filter".
When it comes to google, you really have to keep an open mind. Its easy to read articles and books about SEO and then think you are a professional. The more you think you know, the less you actually do know.
A lot of people get caught up with the vanity of having a high PR website. Ok, you get your website to a PR6, now what? Are you now ranking for your money making terms? If not, what good did it do you?
A lot of webmasters chase links from websites that have a decent PR which is stupid. No natural back link profile will consist of 100% PR3+ links. The fact is, you need relevant, quality back links regardless of the PR. Those pr N/A and PR1-2 links do help and they are easier to get, and guess what, they look a lot more natural.
The fact is, any website is susceptible to punishment by G. It is much harder and much more unlikely that an authority, established domain will get punished, but I have seen it happen.
I have a theory that google will filter a website that it suspects is buying links, and drop them to page 5-6 or something as a test. The test will be something like "ok, we gave this site a penalty, lets see how this webmaster reacts to this penalty" if the webmaster continues to get backlinks, and not lose any backlinks for x months, it will pass the test and we will lift the penalty. If the webmaster starts losing these valuable backlinks, it probably means he paid for them and decided it was not worth it to renew them.