If Amazon ships you a bum unit they have to take it back and eat the shipping cost. Of course if you bought it locally you would be able to drive back and replace it sooner, but who likes leaving the house? I bought my two PS3s and my PS4 at Amazon.
This is the reason I buy most stuff at Amazon now. Whenever I have had a duff item or even ordered the wrong thing they have just excepted the return with no questions and replaced the item.
Best example was a set of Triton AX720's that I bought and about 3 months later the cable that joined the control box had frayed exposing the wires. I contacted the sellers directly and they said I would have to return the unit to Triton directly but they couldn't offer a replacement or refund. Since I bought them through Amazon I sent them an email and they sent a replacement set next day and I just packaged up the old set and sent them in return. That's the kind of service I like.