As per your policy seller has to refund the money to buyer or have to replace.(not sure about this)
But lets take an example...
just imagine a buyer buys a memory stick from us.
We send him a good working piece after due checking.Now he opens a dispute item received is not working and and wants a refund..obviously he has to return the memory stick back to us but HE CAN ALSO SEND HIS FAULTY PIECE BACK TO US(Which he has with him before making purchase from us)....As you might be knowing there
is no serial number on this sticks so you cant differentiate which one was yours,neither you can prove that becuase these sticks are identical.
Think about it and act on this issue immidiatedly than we will restart our listings on ebay.in
