I once new a man who owned a used car lot. One day he arrived onsite to open the lot and discovered that one of his cars had been stolen during the night. He was enraged, depressed, and scared. Later that morning his mentor arrived onsite and I overheard their conversation.
The mentor said "Relax....your lost your first car. And I guarantee you it won't be your last stolen car. Risk is part of the business. Don't take it personally. Keep emotion out of it. Work hard to minimize risk and get better." I try to remember that advice.
Ebay is my strongest and best retail market. Humans make mistakes and eventually I'll make a mistake and so will my buyers. I recently shipped two sets to a buyer who claimed there was only one set in the box when it arrived. I knew for certain I shipped both boxes so I assume a theft at the doorstep, Fedex or the Fedex Drop-off location (Walgreens). I forced myself to stay calm and I explained the above to the buyer. I asked for pics of the box and packing material, explaining that I was going to open a Fedex claim. I told the buyer I was sorry - he paid for two sets and only got one, and I offered to refund him for the missing set. (one set was about 15" long and the other was 10" long - both good size).
About 2 hours later the buyer emailed me. They were at the hospital. The package was a gift. When the kid opened the box and saw the big set he got so excited, he left the other set in the shipping box wrapped in paper, set it aside and never noticed the 2nd set which was under the bed. So they found the missing set and everything was fine. This type of thing has happened to me before because I wrap things very well.