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.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
I don't know about Campy but for years there were low cost Shimano and SRAM components sold by the big mailorder companies like Nashbar that came in bubble wrap in plastic bags.
It turns out these were left-over parts from bike bike makers like Trek and Cannondale that Nashbar bought at a low price and sold them at well below normal cost. The bike companies buy components in lots of thousands packed in large boxes, not individually boxed, for speed of assembly and those that aren't used when a model changes, are wholesaled off.
I once bought a complete 8-speed 105 triple crank from Nashbar for about $35 and it came wrapped in bubble wrap in a bag.