I think used clothing is probably a difficult category on ebay as it can easily be abused by buyers.

I can't see a lot of negatives or neutrals in your feedback though over the last year.

I am wondering if people in this 'group of buyers' have contacted you privately to arrange returns or refunds or have they gone through with actual ebay claims?

I don't think big designers would be employing people to sabotage your clothes. You're selling some known brands but we're not talking hundreds of dollars plus your items are not new. 

What i have seen is some companies are more inclined to get listings removed completely, especially if the listing is for something new, if the ad is not through an authorised seller. All that is legal and above board.

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?

"I been told that big designers employed peoples to do that to stop seller's from selling there brands cheap on ebay( used clothes) "

I think it highly unlikely  this would be true for used  clothing

"There are group  of buyer keep buying the clothes and deliberately damaged it and return it  back ask for refund"

Also buy items and leave negative feed back without contacting me"

What makes you think the 'group' are linked ?

Similar names ?

Similar  or same addresses ?

Look at their feedback left for others.

Are they leaving negative or neutral feedback for other sellers ?

If you think it's deliberate, report them to  eBay, mentioning all the IDs that you think may be in a 'group'

https://www.ebay.com.au/help/policies/rules-policies-buyers/buying-practices-policy?id=4374

Or go to Live Help and when you get to the automated assistant, ask to speak to a supervisor.

Any one had this experience  ?

Whether this is abusive buyer behaviour   in your case I don't know but yes, it can definitely happen

Good luck with it.