The reason I buy almost everything I buy online via Amazon is that if it is sold by or fulfilled by Amazon and I order it I will get it exactly when I specify 99% of the time (generally two days from order, unless I really want to pay the extra ~$4 for one day on top of my Prime membership). Every other shipping/buying concern is completely secondary for me -- sellers who sell on Amazon but don't fulfill via Amazon offer very little benefit (ease of one-click shopping is nothing compared to the nearly guaranteed consistent delivery, at least for me).
I actually wish more companies than Amazon (and Newegg) would handle this sort of consistency better because I'd love to spread my dollars around more, but those are the only two that have earned my trust in this area. Every other online seller I've used might as well have "???" listed for shipping times because that's what actually happens.