This is why Amazon handles the logistics for so many third-party sellers rather than simply buying their merchandise and reselling it. They get the little guy to assume all of the risk. Unsold products? Your problem. Some kind of defect? Your problem. All Amazon is on the hook for is shipping (and warehousing, although the cost there is a bit more abstract).
And when they are handling the logistics, they also get the benefit of just giving a full refund for whatever reason they like. It's nothing out of their pocket, and it makes them look good to the consumer. So, why not?
They also get to look at all that sweet data and if any one product or group of products starts to look too good for those small 3rd party sellers, they aren't afraid to just drop right in on top of everyone and push their own product to the buy box everytime.
After which they will drive away most of the 3rd party sellers before starting to creep the price upward.
