A slightly larger battery combined with a plastic case pretty much negate the better specs. They're basically the same specs and some may find the 5 more desirable because it has a metal case, especially in now discontinued black and slate.
Bigger battery and better HD front facing camera. I agree the metal is probably more desirable to most people that never had it. From experience, I would choose the 5c over black/slate 5. It scratched way too easily.
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By the way, this thread has me really worried:
http://forums.imore.com/iphone-5s/268387-activation-locked-iphone-dfu-mode.html
A guy in the same boat, and he LOST the case with eBay!![]()
I hope it works out for you. I would keep hounding the seller. I wouldn't stop emailing him to the point that he gets annoyed. Have everyone you know flood his inbox lol.
As you said earlier though, 8 years on ebay and 100% feedback he might not be a bad seller. Give him a little time (24 hours).