Points to a couple of the ways that people taking online courses can cheat by paying other people to do the work for them. However, I don't think this has anything to do with whether the courses are online or not. This is about how to crowd-source cheating, whatever the modality.

I have come across the results of some of these when marking work in both face to face and online courses. They are sometimes easy to identify. Many academic have signed on for such services and I have quite frequently received reports from others who have found my students using them, as well as finding a few myself. The detection process is helped by the fact that many who provide such services, ironically, tend to take shortcuts and re-use not just their own work but the work of others. 

My own approach in courses that I write is to make this kind of cheating too expensive to be worthwhile: linked assignments that are inherently unique, therefore requiring assignment providers to take most or all of an entire course, can significantly reduce incentives for cheating, especially when progress has to be shown through the course so there is no point in hiring someone who already knows how to do it. It's not infallible, but it's less fallible than exams, where over 70% of students in North America admit to cheating. When combined with a process that makes it likely that a tutor will get to know a student fairly well and a process that requires students to share work with others, leading the benefit of many eyes (honest students are usually affronted when they find cheating among classmates and willingly report it), it is heading in the right direction.