Use the Good Courier Service. This normally makes you on safer side. DTDC has one option Open Shipment service. See for it. Its all how you talk to your courier agent near to you. He can definitely have control on thefts/missings.
We can trace out the things easily with the help of your direct courier agent and with the help of destination courier office (not agent). Work with them.
These things can't be revealed here as this may provoke Buyers or Sellers to cheat further. Every cheater leaves evidence and every evidence has a loophole.
Yet few things to jot down, wherein you can think yourself wisely and get the solution.
Case 1: Buyer is genuine and truly received empty parcel and Seller is genuine and truly sent parcel with item.
Solution: S: Talk to courier agent at your area. At the same time give support to B by telling him you are tracing facts. B: give sometime for S to trace things. If this is happening manytimes, leave the courier immediately and file a case.
Case 2: Buyer is cheat and claims empty though received parcel with item.
Solution: S: Give B a chance to rethink that he is doing wrong. Also, talk to courier agent at Seller area and destination area. Boldly say to B that he is caught and you are taking needed action on his id.
Case 3: Buyer is genuine and truly received empty parcel and Seller is cheat and sent empty parcel.
Solution: B: Give a chance to S to agree his mistake and have refund. And raise a complaint, refund etc;
Case 4: B is a competitive S of your items.
Solution: Raise a complaint to eBay and they will easily trace him using many many things like hidden data, ip, cookies, pattern of access etc; You may not see immediate action, but sure that it will be actioned.
Ref: S == Seller; B == Buyer.
Note: Remember that B would be true. Don't see his past purchases/feedbacks he got. S will have only option to give positive and B may be so cruel. Buys few items just to act as he is genuine.
TIP: Always stick the eBay invoice copy, this definitely reduces theft or use genuine courier.
Interesting: We have seen few B's with around 100+ feedbacks and who try to play with S to get free gifts and free products twice luring S for +ve feedbacks.
Remember: This is online business and we need to fix our mind that these are normal situations and we should stand with such things. Go on, go on learning day by day.
Regards