My own view as a writer, for what it's worth:

I probably would not accept a job to write someone's dissertation. If the case was like the one linguaga described above, where the client had actually done all of the relevant research and just wasn't able to do the writing well enough by themselves, I might consider it.
In any other case, the pay probably just wouldn't be worth the effort. Term papers, even for graduate courses, tend to be pretty narrowly focused - in the areas of the humanities in which I work, at least. If I'm doing a 10-20 page paper for someone, I'm willing to do an in-depth reading of a few sources, e.g. if the term paper is a critique of a philosophy essay or something that needs really thorough analysis. Or, alternatively, I'm willing to skim 5-10 sources if the paper needs to have limited survey of the relevant literature.
For a dissertation, however, you're pretty much expected to have reviewed *all* of the literature that is relevant to your (narrowly defined) topic. I mean, if you're writing on the impact of the Portuguese language on Lord Byron's rhyme schemes between 1809 and 1811, you're going to be reading virtually everything that has ever been written on that topic. It can be tough to find all of it sometimes.
Even if I'm trying to severely half-a55 the effort, I'm going to be doing an awful, awful lot of research. Far more research per final dissertation page than it would be for a term paper. Now, if you're willing and able to pay a lot (and.. I mean, a LOT), maybe you can find someone who is willing to do the research that is necessary for even a sloppy dissertation. Think of it this way: It varies by subject, but in my own field at least, the dissertation is something you work on for about two years (after having already studied in the field for 2-3+ years at the graduate level). So how long is it going to take someone to write this dissertation for you, assuming they put in at least some minimum effort? And how much would someone with the minimum level of ability required for this expect to earn for that much of their time?
If you're considering hiring a writer, I'd suggest working with someone directly. Essay writing websites take a good chunk of the essay price for themselves. Nothing unfair about this in my view; they're taking care of all the business end so I can focus on the writing. But when it's a very long, involved work like a dissertation, it simply won't be worthwhile for a legitimate writer rate of pay they get through the websites.