As Google clearly states (with their actions), bartering for links is fine as long as money is not part of the exchange (or if there is editorial discrimination and relevancy when it is). What is the guiding principal for bartering links? Massa said:

When do other people WANT to accept your link request, publish your article, run your press release or accept your submission?

When it does something for them.

Either it makes them money, saves them time, provides added value to their visitors or they believe it makes them look good or smart or benevolent to their visitors, their peers, their friends, their relatives, to the search engines, award sites or just about anyone that can make them a buck or stroke their ego.

So, the absolute best chance you have of getting that link is to cover as many of those bases as possible at the same time. When you can satisfy some need, want or desire of the webmaster, the visitor to the hosting site and it makes the search engine look smart, BINGO. You just hit the SERP buster hat trick!

If you are going to sell links, you can cloak them as being AdSense ads to lower your risk profile, because if it's Google its got to be Good! Or you can require the link purchase be wrapped in a guest article or some other format that does not look like a link purchase. :)

Update: Gab Goldenberg wrote this detailed post on real sneaky text link ad disguises.