One-Way Links

On a pure value standpoint, one way links (sans any linking schemes) are the most valuable links you can get. This is, in most circumstances, a link to your site from someone that finds your content valuable, doesn't ask for a link back, you don't link back to them on your own.
If you have good content, this type of link isn't all that rare. The best link marketing is creating a valuable website that is informative, helpful and educational. Of course this isn't proactive marketing, which is why you see a lot of options to purchase one-way links.
Without a doubt, search engines frown on purchased links that are not clearly defined as such both visually and/or using the no follow attribute. Purchase links at your own risk. If you are using a broker that claims to keep their purchased links hidden from the search engines, keep in mind that the search engines can easily spend money building sites and purchasing links from these same networks. The hidden network is no longer hidden from the search engines and they might not even be aware of it.
Not all purchases links have no value, but only so long as the link flies under the search engine radar. This can rarely be done by purchasing links from a broker, but can be done on a one-on-one situation.
Links are an important part of your optimization campaign, but not every link will provide you the same value. Aside from worrying about reciprocal links, link wheels or even purchasing one-way links, go outside of the box of "get me a link" and think about providing value to your visitors that is worth linking to. Once you have that, then consider ways to get the word of your content out to those that are most likely to link, become customers, and spread the word.