Good point. When I was an undergraduate we received dire warnings about cheating and I only remember one instance of anybody being caught doing so - two of my friends had colluded on an assignment. That was in about 2003, so well into the internet era but still pre-smartphone, pre-Facebook and probably before the internet became the source of aids to plagiarism it now seems to be. Do you find it a harder battle now than it used to be, or is t only as bad as it's always been?
A) I am not in a discipline that gives many tests, and my pedagogical method is to give as few tests as possible, as I feel essays are a better opportunity for students to demonstrate their ability to analyze and develop their ideas,
B) Of course, it is possible that students are buying these essays, but I would probably never be the wiser,
C) If they are buying these essays, some of them are not getting great value for the money. (In other words, why am I not grading all A essays?)
Of course, I guess you could counter point C by noting that ballroom dancers often pay 3K-5K for dresses, and yet one sometimes sees ugly dresses on the floor that make a poor argument for marking the dancer first, so maybe ugly prose and weak analysis that makes a poor argument for giving a paper a straight A isn't all that different.