I worked at UPS....
....about 20 years ago. Absolutely no loyalty.... Gawd I hated sorting packages in the middle of the night. But I was in college and needed the moo-la. Picture this:
The load shifts during transit, leaving a pile of boxes against the trailer door when it's time to unload it. They force it open, crushing a few boxes, then a dozen more fall out 4-8 ft to the dock, or even bounce to the ground another 5-6 ft down. This was not uncommon in December, when the trailers were overloaded.
Imagine that your heavy receiver or amp is one of those falling boxes. Pack it to survive a 10 ft drop! And if it is a heavy item, make sure that the outer box has cutout handles on it to encourage safe handling. If they are available, someone is going to use them on a heavy box.
Nowdays I ship vintage gear by Fedex Ground (or Express if the receipient is willing to pay for it). I guess it is better, but who knows!
The problem with buying gear on ebay is that the seller has only their reputation to inspire them to pack really well ....and usually a cost incentive to pack cheaply. If you are in doubt, pay the seller extra for boxes, bubble wrap, sheet foam, etc. when you purchase a valuable item. Let them know you are concerned and tell them how to pack it.
RobV