I will go so far as to tell you this, passing off someone else's work as your own is considered plagairism.
It is NOT illegal or in any way a violation of any rules for the writer, so long as he/she sells the work with the codicil that it is to be used as a reference. Of course, the writer is not there to monitor what the buyer does with the work once it is sold.
But if you, the student, uses someone else's work as your own than it is plagairism.
I would never buy from one of those websites, because you do not know the origin of the work and it's entirely possible it's been sold and used by others, hence, not original, and therefore may be picked up by software designed to catch plagairism.
Most sites cater to common first and second year introductory courses. Those are pretty easy writing to the experienced.
But the truth is, if you have to patronise one of these anonymous websites you're straying into dangerous waters.