There are two possibilities here:

1) Amazon has unwittingly bought a batch of stolen phones

2) Somebody has reported a phone stolen to claim on insurance and then returned the for a refund, it either being repacked by the retailer or sent back for factory repacking.

Phone insurance fraud is rampant here in the UK, a recent estimate said that the majority of phone robberies were actually just so that people could get the latest phone, and some phone dealers/sales staff were encouraging people to report a fake robbery so they could get the commission on a new phone.

Gareth