this one is pretty clear to me. USPS screwed up, so they should be held responsible. Because they won't/refuse to/whatever, i don't see why eBay should be held responsible for USPS mistakes. Yes, I work for eBay so if you want to say I'm biased have at it, but that's my unbiased opinion, and you know I'd freely admit (and try to help) if I believe eBay were in the wrong. By the way, these same types of things happen with Amazon with the same results. In other words, if the facilitator isn't the originator of the shipped package there really isn't a lot that can be done on their end. And if Amazon, eBay, whomever made amends on behalf of USPS/Fedex/UPS mistakes every time that wouldn't be very smart business practices. Companies like Target or Walmart or whomever are in a different/better position to do that because they can easily replace/write off the missing product, which they might have even produced.

So maybe next time buy your vintage bike parts from them

edit:

btw, what I could see happening in this case would be having the seller get ahold of eBay, explaining the situation, and I'd be willing they'd probably credit the seller back some fees to help offset the cost of re-shipping. You might mention that to the seller. it's worth a try, and perhaps he could pass on or share the savings to whomever ends up footing the bill to fix the USPS mistake.