I've heard of it once, but with no substantiation. A reputable seller will mave lots of sales and nearly perfect feedback. The pictures alone should tell the story. Faking Campy parts would not be easy. Many sellers take pictures with the parts in the factory boxes.
FWIW, some sellers may have purchased the parts from Europe and they may have been shipped without the boxes in order to fit more parts into a flat rate sized box. No big deal there.
I'm selling some Record 11 50/34, 172.5mm cranks on E-bay right now. I took pictures out of the box, but noted that the cranks would be shipped in the factory box.