That, and there's re-manufactured and refurbished returned products that sellers buy in quantity and sell for discounts. As long as a seller properly identifies the source I'm fine with that as long as the price reflects the source of the item and it's in new or like new condition.
I try to find PRIME items that offer 2 day delivery. Those are stocked in Amazon's warehouse instead of being shipped from a seller's garage. $99 a year for Prime is worth it if you use the video, music, free 2 day shipping, and other bennies that come with Prime. I pay that much to Netflix just for the movies.
Amazon recently bought land and is building a humongous shipping warehouse in Macon. I'm hoping for some sort of benefit with the warehouse being about 15 miles away from my house. They might just provide same day USPS pickup and next day delivery in cases where the product is turned over to USPS the night of the order and hits my mailbox the next day. My luck, it would go to USPS Atlanta in a truck with all the other stuff and be trucked back to Macon the next morning!
My wife and I share a Prime account ... at times we've ordered just 1 inexpensive item and had it in two days ... Amazon must lose money on us at times, as processing, handling and shipping costs likely exceed the cost of the time we purchased.