What kind of margins can I expect buying big quantities (100k/1M) in tech products directly from manufacturer?

Walmart calls it "Pay from Scan." Also "Guaranteed Sales." Sam's Club calls it a "Road Show."

Selling any product including "iPhone cases" on websites with high competition like Ebay is surely not an easy mission. There are hundreds of sellers on Ebay and other websites who are already selling cases. If you need to get some sales, you need at least one of the following: 1. High reputation, reviews or feedbacks on the website that you are selling on. 2. The product that you are selling must be unique in some way or simply not found anywhere else. 3. If your products are featured at your own website, then that website of yours must be ranked properly online and marketed decently around social media networks. In your case, a possible approach would be to create your own website to sell your products on or work as an affiliate marketer for other big sellers in exchange for commissions. Depending on your budget, you need to draw the right plan around. If you have a good budget to invest in your business, then creating a website for yourself is a must. Otherwise, you need to rely on other websites and seek to provide unique or special products to get an edge around. Hope that helps!

I live in Brooklyn, NY, which is a really fertile ground for small business retailers. My experience speaking with a lot of the shop owners is that they simply don't have the resources to maintain an online presence. Just keeping the brick and mortar operation running consumes most, if not all of their time, especially if the are also creating and producing their own products. For many that I do see take a shot at e-commerce retailing, they may get Shopify/Squarespace site set-up, then just run out of steam to keep it going. One innovative, in-between approach that I've seen one of my favorite shops take, is to actually focus on using a single social media channel, Instagram, and maintain an e-commerce function through it. (See @peopleof2morrow on Instagram)

That depends on how differentiated you are from existing products and if having your product next to a dozen competitors on a shelf will help sell it. Do customers need to touch it to see why it's different/better? If not, brick-and-mortar might not be necessary, at least initially. Warby Parker overcame this issue by making their glasses easy to return. Try on a few frames and return the ones you don't want.

I would offer them a proposal to reveal their source to you in exchange for a small percentage or fixed amount of each unit ordered. You would also need to structure the offer in such a way as to guaranty that you would not compete with them or circumvent them, if they agree then you would need a legal contract drawn up between you and them