Selecting an implementation partner for your eCommerce platform is hard. Based on a limited amount of knowledge a decision has to be made between a number of providers whose initial pitch is relatively equal. Assuming you have chosen for Magento (solid decision), here are some pointers for selecting your eCommerce Magento partner:

1. Choose a partner who has solid experience in developing Magento. Magento is, at the time of writing, still relatively young. The learning curve for development in Magento can be higher than in other eCommerce software. A systems integrator with a lengthy history in other software but none in Magento will have difficulty getting up to speed in Magento in time to start and finish your project in a timely manner. A younger company that is more versed in Magento might be a better choice.
2. Systems integrators from the “old world” of very steeply priced commercial licenses are still adapting to the professional but open source Magento software. Be smart in prying through vague explanations for costs,
3. Do not get pressured in making your decision. Conversely, also do not pressure your provider by having them commit to deadlines while you delay your decision making. Keep the process fair and the quotes and subsequent relationships will be healthy.

Of course, a general rule is to go with your gut feeling, especially for not selecting a partner. If it does not feel right, do not do it; if it feels right, then rationalize the rest of your decision.