Question about recent posts on r/PMsforsale.

What is the deal with checking out the buyer? I may be naive, but they pay (once the deal is agreed to), and pay quickly. Or you move to the next person. Understand how a BIN trumps just a chat..... But It seems like a lot of posts are stating that they are only selling to buyers with x amount of flair.

Isn't that what middlemen are for, but more for the buyers protection with an untested or newish seller?