There's a guy whose job is to look at incoming packages and throw them into appropriate routing bins. And he has to find and scan the bar code on all of them. All day. Every day. That's all that guy does for eight straight hours. Once in a while, he gets it wrong, and the package ends up in the "go-there" bin instead of the "stay-here" bin. They catch it at the "go-there" place and send it back--that's the check and balance in the system. It's mildly annoying but it usually works itself out in a day or two.
Occasionally, that guy forgets to scan the item, or the bar code becomes garbled for some reason, but even then the package moves through the system even if the tracking is lost. I've had a few packages that disappeared in the tracking but arrived on my porch on time. If Fedex loses the airbill, they use the secondary bar code on the package, and if that is garbled, they interpret the address and create a new barcode. But that also takes a day or two.
If my shipment is mission-critical, I pay extra for an express delivery, but then I don't buy mission-critical stuff on ebay very often.
Mailorder specialists like McMaster-Carr put the routing information and shipping-service labels on the packages themselves and those packages always find their destination quickly in my experience. But individuals selling stuff occasionally on ebay is the opposite extreme--all the routing information has to be put on correctly by the guy at the shipping desk of whatever service. USPS has actually improved in the last few years because 1.) they print new address labels for packages and don't depend as much on handwriting, and 2.) they require the sender to confirm the destination address on the little screen at the post office before they print that label.
Rick "doesn't want to be that guy sorting packages and thus is grateful it's usually done so well" Denney