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 P

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.