I am personally still learning the art of grading and I think the majority of people still can't grade properly. (Forum members excluded)
I find it hard to buy notes when people say they are UNC but when you get them they are EF/AU and it is very obvious once you look at the note for 5 seconds.
I try to only buy UNC notes from rep dealers/sellers or I would buy a BCS/CCGS graded note. If not I would never pay even close to Book from someone I didn't trust. I have sent back many a note that was "UNC" to the seller and not even close when I received it.
I trust BCS as almost every note has been reasonably graded. I would never buy a PMG note as I have seen to many overgrades from them.
But it is hard to trust people when they don't know how to grade. I mean if you hold a note up to the light and see multiple lines... its not UNC
its not that hard.
I personally have gotten several note graded by BCS as the cost is not that bad and it "stores" the note as well. Its not like you can't cut it out of the holder again.