@varunb, One thing you didn't post the pics of any of the accessories, and did not even mention the I/O shield in your original selling thread. You are supposed to post pics of EVERY accessory you sell. Secondly are you 99% sure or 100% sure that you shipped the I/O plate? If it's the former (as you told him on phone) then it's your responsibility to ship him that since you yourself aren't 100% sure.  And yeah if he is willing to cough up for the I/O shield, then do send it to him in spite of the fact he has been abusive or lying as you claim, if made the deal with him after all.

@funskar, When you bought the case why didn't you check out the listing of back plate in the original thread? You should have also insisted for pics of every accessory before coughing out for the deal.
Secondly you should mind your language and should never post personal details in public. That's an infraction worthy behaviour no matter what the case. You can PM those stuff to any of the forum staff, if needed. Your behaviour hasn't helped you one bit, in fact it had made the case worse for you.

Also the forum cannot do anything here really. It's only a medium through which the deals go through. There's not enough evidence to take sides or action against any of the members, and nothing can be done. Keep your discussions through phone, e-mail, etc. The forum isn't eBay and is just a medium of communication here. You won't be arguing in MTNL office for a deal taken place in phone for instance.