I've always taken it as a proxy for quality. My thinking was that Prime means it must ship from Amazon (I realize now marketplace sellers can qualify for the Prime flag if their shipment is free and fast for Prime members). And then it follows that shipped by Amazon means they stock it, and are responsible for returns. Therefore, not wanting the expense of excessive returns, they would probably have a system to ensure quality merchandise.

Of course, none of that thinking is accurate, but it appears to be logical on the surface.

And yet many consumers conflate "Amazon Choice", "Amazon Prime" and "Sold by Amazon" and think they all mean roughly the same thing, some stamp of approval by Amazon. But Amazon has no incentive to disabuse them of that notion.

And it's not even that any more. You ALWAYS have to check the guaranteed delivery date. ALWAYS. At least half the stuff I start to order -- even with the "Prime" label -- will take longer than 2 days. This situation has caused me to start shopping retail again, whenever I can. It bites me EVERY time I don't specifically check, and stuff winds up coming on a slow boat from China. Prime literally means nothing to me anymore, except their video service (which has a pretty good selection).