Amazon Delivery Estimate Question

Im purchasing an iphone case on amazon The one im buying cost 8.49 via amazon but i see that the seller is a certain person that also sells it on ebay. I also have some 3 dollar and so change on gift card so when i purchase it, i could get it for bit more than 10 dollars due to shipping. It says delivery estimate is march 7.
However… i saw i could buy the same item for 5.50+2.99 shipping and thus its same 8.49 price. Differene however is they say shipping estimate would be march 10-13. This is the same seller on ebay.
So if i buy it from seller and shipped from seller, its about 5-6 dollars less. But isn’t buying it from amazon going to come from the exact seller as well?
$8.49 plus tax

$5.52+2.99 tax so $8.49 total

Can someone explain this? So if i buy it 5.52 plus tax so 8.49 total and thus 5-6 dollars less, im going to get the item few days later? It does say estimated shipping is march 5 or march 6 though. For the amazon one it say estimated delivery is march 7.

The item could either be shipped:

  1. Directly from the seller
  2. From Amazon warehouses (it is called Amazon fulfillment) [the seller would have shipped part of his stock to Amazon warehouses so they could fulfill orders]
    So this could cause differences in delivery times (the seller is 2,000 miles away but there is an Amazon warehouse 400 miles away).

The numbers Amazon posts are simply estimates and may not be provided by the seller and should not be taken as definitive.

I would usually buy from the cheapest option.

The seller might be very conservative in her shipping dates to prevent something from getting there late, which would result in negative feedback. If it gets there early, people usually remark on the fast shipping in their feedback, and that’s good for her.