On Amazon cheapest used iPhone Verizon 4s is $305 - no guarantee on the condition.
Amex won't give you a warranty on Used items, and even with Amazon - if you buy used, you are in the mercy of the sellers return policy.
Amazon A-Z is only on FBA items.
I would agree with you - that at least by buying on Amazon you are covered with Amex return protection.
IMHO - Unless the seller says "in 100% mint condition, was always in a case" don't book on it to be in good condition.
#my$0.02