I don't get a lot of spam comments on this blog. In fact, I get almost none at all. The very small exception to this rule is a specific type of comment, which may well come from a human rather than a robot commenter (which is how they get through the spam filter). They're all on one particular post (this one) and they all, in slightly faulty English, praise the post and then put a link to an essay-writing service (not always the same one). I'm fully expecting to get similar comments on this post now.
I checked out these sites once, just to see what exactly they claim to offer. They're invariably quite badly written sites, which doesn't exactly instill confidence (although I suppose if you need to buy essays, you're not going to be the type of person who notices or cares about such things). Anyway, they all say that the essays (written by postgraduate students) are not to be passed off as your own work, and are examples only. Why you would pay a fairly large sum of money (not a fortune, it's affordable for a student who really wants to pass, but not a trifle) for an example of a good essay, I don't know. Obviously, they know that people are handing these in as their own work, which is plagiarism, but they cover their backs by saying explicitly that they aren't to be used that way - then they can say, well, it's not our responsibility what people do. We told them not to and they did it anyway.
But this kind of plagiarism must be the hardest to detect, or at least prove. If you copy big chunks off the internet then it's easily spotted and easily proved. If you 'forget' to reference an article, you'll get very swiftly found out and again, can easily be shown to be plagiarised. If your work is very similar to someone else's, you're both going to have some explaining to do. But if you hand in a bespoke essay, containing original research and writing, just not your own, what to do? Your tutor might notice that your writing style is suddenly completely different, or that your punctuation has suddenly improved or your ideas are suddenly vastly more sophisticated. They may be pretty certain that it's not your work. But how can they prove it? It gets to be a big, complicated process where people have to look at your work and you have to show that you really wrote it and explain the improvement in your work... (you can of course buy an essay that gets an average mark, which is a bit smarter, but then why would you?).
So this is why these comments all get marked as spam as soon as I spot them. I won't have them, and I think the services advertised within them should be outlawed. Although I may be looking for a job from them if things go belly-up with the PhD.