Which iphone model are you looking at? some had abysmal power ratings when new. My 5 and 6 were both power hogs. I have never had an issue with amazon customer service, ebay customer service or for that matter paypal are not my favorite. If there is an issue with the purchase there are a few more hopes to jump through, contacting the seller, waiting, escalating, calling paypal(they only have phone customer service, no email or live chat).
I have bought a couple off of Swappa and have been happy with the phones we have purchased. The protection plans are reasonably priced as well. Just another option that might be worth checking out.