Doing business online or having eCommerce business have failed at the hands of poor design, development and effective marketing strategies. In eCommerce some will focus on Design while others will optimize the engine that runs it. Some will focus on SEO, ad others will prep for conversion.
The Problem, then, is that you can't possibly know what to expect. Every developer is different and so, too will be their goals for your eCommerce Business. This means you need to prepare a list of questions to ask so that you are not left in the dark about what to expect.
The last thing you want is to hire an Agency, pay a lot of money, spend a lot of time and receive a product that hurts your business. I'm here to make certain that such things do not occur..