Apart from the obvious that using a term paper mill constitutes an act of academic dishonesty, there are other reasons to avoid them.
First, the quality of the work on these sites is often stunningly bad. Students who post their work there in order to gain access to other papers on the site have no incentive to put significant time or effort into these papers.
Second, these essays are almost never going to be a good match to the assignment that your instructor has asked you to do. How can they be? They were constructed for another assignment in another class! The time and effort it would take to “adapt” their work for your assignment would best be spent doing your own research.
Third, the content of these web sites is easily detected using online search engines and plagiarism detection software. It will be remarkably easy for your instructor to discover the source of your paper if you decide to turn to these services. You will be risking serious academic penalties should you choose to use a term paper mill.
Finally, some students suggest that they use term paper mills because they just want to get an idea of what a good essay looks like. Fair enough, that’s a noble impulse! But what makes you think you will get a good example from an anonymous term paper mill? If you need help on an assignment and want to see an example of work that meets the instructor’s expectations, go see your teacher! Professors often maintain a file of student work that you could read as an example to gauge the requirements for excellence in responding to a particular assignment.