That will depend a lot on the complexity of your build. If your ecommerce solution uses an open source framework, then a back-end developer may be more essential than someone focusing on design (though you'll likely need both). If you're using a SaaS platform, you'll probably need more front-end work than anything else — though you may also want someone well versed in building APIs. While most SaaS platforms have a relatively robust set of apps with pre-built integrations, further complexity is best handled by an expert.