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!