The prices on Etsy are on the higher end in most cases anyway so you wont have much of a margin unless you can strike a deal. I would not try doing it without the Etsy shop owner knowing first because there is an app out there that helps etsy sellers see if their products are being sold elsewhere. They get all pissy if you do it without them knowing. It would be best anyway to get to know the supplier of the products and to try and get a discount anyway so it's best to reach out t them first.
There is no reason why this would be any different to doing drop shipping with any other supplier.
Why would people shop through you instead of going to etsy especially if you are stating its a product from that shop. You will have to give an incentive to them to shop there. Why would they trust your shopify site over a multi billion dollar marketplace. If you get sales or not is 100% down to how much effort you put in. You cant just slap up a few products to a store and expect to get sales.
I'd like to open a shop instead