No, I have never had anyone tell me such nonsense 

Do you really think that big design companies are going to bother themselves with trolling eBay to look for used clothing? 

How do you know they are a 'group' of buyers?  (indicating they are somehow connected/know each other?)

Have a look at any of the posts here where buyers have been busted on social media wearing the clothes they bought from an eBay seller and then opened some kind of dispute after they had worn them for said photos) Are those people also employed to do this?

Have you blocked the buyers that do this?