I'm looking at pricescope sorting diamonds. I know GIA doesn't grade cut on fancy shaped diamonds so how do they determine how well a diamond is cut? They do range from no grade to ex to poor? I know a lot of online suppliers don't own the diamond or even see it so where would they get the "cut grade info"

Online would get it from the people they are buying it from to re-sale.

I would never (did I say never) buy diamonds online because you can not have them inspected prior to purchase. If one is discussing a high end purchase, they should be examined by an independent person to determine value.