There is a very basic equation that you have to meet to have a chance of being successful in retail, and it boils down to being able to find the product at a price, that when all costs are factored in, will still deliver you a margin worth the effort.
What you have highlighted here is the fact that the equation isn't adding up, and therefore you now have to work out why. First off you need to look at your purchase price, how competitive is it against other suppliers. Of course the best way to get the best supply price for any item, is to purchase in high volume.
The more you buy, the cheaper the cost per unit will be for you (within reason of course)
That's where I would suggest starting, after that you are just open to natural market factors, and there are a lot of them on a site such as an eBay. One for example, if you have a competitor buying at your price, but selling at a extremely small margin, then you may find that it's not worth your effort and outlay of cash to try and compete.
Now I have ignored the fact that you are talking about iPhones deliberately, as the above advice goes for any product, but that's one of the hardest markets on earth to get into.
If you do the research, find that you ad still unable to compete at a level you want to, then you need to start opening up and researching other product avenues to find a market that is still available at a level where you feel comfortable competing.