I guess I've been lucky--in the hundreds of purchases I've made on eBay, with the vast majority being sent USPS, the ONLY ones I've had damaged enroute were damaged by commecial shippers. From your description of the lack of packing care I'd assign a lot of the blame to the seller as well since no matter who you ship with you can expect packages to get stacked on top of each other. A sturdy box is a must! I do hope your seller learns that lesson instead of blaming it all on USPS and not improving his ways.
But still, no matter who was at fault, it is sickening to see an irreplacable hat or other item arrive destroyed due to lack of care.