To make sure that you choose right, the first thing to consider is to check their own website! If they can’t impress you with their own work, there is little chance their work will please you.
Then, look at their portfolio and see if you like what you see. Have they done work on a website like yours in the past? If yes, and you do like the work, they are in a better position to handle your website.
Testimonials are another great way to peek into the company’s reputation. If the company is forthcoming with contacts you can talk to, this is a positive sign that they are confident about their work.
Talk to the shortlisted companies and tell them your requirements. A good company will be ready to discuss your requirements and will be open about the kind of solutions they will provide. For an e-commerce website, they should design it in a way that lets you add, change or edit products and descriptions.
Discuss upkeep and maintenance of website in clear terms to avoid ambiguity later. Monetary terms should be clearly laid out for each aspect, including long term association. Also, clarify whether suggesting changes is acceptable once the website is ready and will there be an extra charge for the same.
By being clear about your requirements and being with someone who understand them and will readily translate your vision to get effective results, you are poised for great success.