My wife and I both sell quite a few things on Ebay and we both have worked in a USPS mail processing facility, she retired, I am still there, but my retirement is about two months away. Anyway, I get to see many packages get damaged during processing. Sometimes the contents remain with the package, sometimes they don't.
It is important, if you are shipping something, whether by USPS, UPS or Fedex or some other shipper, to package the item securely so it does not shift around inside the box during the processing. There is very little human handling of packages, or letters for that matter anymore. It is all done by machine which don't read your fragile, do not bend, glass, etc messages. The machines only look for an address, they also weigh and measure the package. I recommend placing an address label with the delivery address securely attached to the item inside the box you are shipping. If you can also attach a copy of the tracking barcode, that is even better. Then, in the unlikely event your contents get lost from it's box, at least the shipper will know where to send it after re-wrapping. If you don't have an address label on it, it will probably end up in the lost mail auction in Atlanta, sold to the highest bidder.
It is also important to securely attach your shipping label to the outside of the package, don't wrap your box with wrapping paper or kraft paper which rips and tears and lets the address be lost. Use the shipping labels the shipper allows you to print online, don't hand write the label using a ball point pen which can barely be seen.
Of course, none of these tips will help with things a shipper is sending to you!