I’ve purchased several articles but have not used any plagiarism checker.
I have not published nor used it on my blog yet. When I do, I may hire another seller to check it out for me.
Copyscape isn’t always reliable. A seller will get a genuine article, set the word document to another language such as Russian and replace all the letters which are similar. Then they will set it to another language, such as Romania and replace all the similar letters, on and on.
They read it to make sure it still makes sense in English, then push it through copyscape themselves. They pass it to client as the last step.
You end up getting sued for using someone else’s work.
Someone (a seller) wrote about it in an older thread when a buyer who swore it passed copyscape.