DHgate.com complaint: Struggling for Refund

Hi my name is Mohammed and I’m a retail seller in the UK. I bought USB storage devices from one of the seller on DHgate. After some time all of the buyers who purchased these USB sticks start making complaints about it as they were not as described. The seller described them as 128gb 256 gb but in real they were on 32gb and doesn’t hold data more than 32gb. Therefore I contacted the seller and he said return them all for the refund. Now it’s almost 2 months passed away that he received the items and I have a valid proof of postage for the returning items but he just make excuses and don’t give me my money back as he promised.
I contacted DHgate customer service about more than 10 times and they said they can’t do anything as the status of the order has been closed. I provided them all the proofs but they just give me a very Silly solution that make the seller to give you a statement in which he says I’m agree to give you this much amount as a refund. I did it actually but the seller keeps making excuses. Really disappointed as I’m a VIP buyer on DHgate.