The purpose of any ecommerce website is to sell either services or products. The thread above mentions SEO and I cannot stress the importance of ensuring that which ever platform you choose, you are well informed about it's SEO capabilities. If it's open source, I would certainly recommend using Wordpress with WooCommerce and Yoast SEO plugin. WooCommerce is capable of handling services or products, whilst adding the Yoast SEO plugin will enable you to ensure that on-page seo ranking factors such as heading, page titles, meta descriptions, URL etc can all be manipulated to suit. Using Wordpress, you have at your disposal a world of plugins that make running a website very easy and cost effective. Websites such as Paypal, marksandspencerforbusiness.com and Vogue.com all use the Wordpress framework so it comes tried and tested.
If a hosted platform is preferred then I would recommend Bigcommerce. I have many clients using this platform, again primarily because it has been designed with SEO in mind. Whilst you do not own the site files, you will gain all the benefits of product updates, technical support, PCI compliance, product inventory management at no additional cost. One very nice touch is the abandoned cart saver where you can choose to send customised emails to shoppers to encourage them to complete the transaction.
Again from an SEO perspective, make sure that your products allow user generated reviews as this will add vital content to the individual products pages which are often associated with thin content.
I would however like to add that SEO should just be a part of your decision making process. You need to make sure that your ecommerce platform is mobile friendly and you personally check out the platform on your own mobile. Our stats show that nearly 80% of traffic to websites now comes from mobile and tablet devices, so whatever you do, make sure you fully understand your target audience and their preferred browsing device.
Another point to raise is to ensure that your platform allows you to build data lists and segment them accordingly. 96% of website visitors do not buy on their first visit, so you need to be able to create call to action and persuasive landing pages that seek to gather visitor data. Your platform needs to be able to segment this data, so you can send tailored and bespoke communications based on the UX.
Whilst Demos are useful, they often come with little support or you are too scared to ask. I would ask for a member of the support team to guide you personally through the software - that they you can ask them to show you specific features and functionality.
Feel free to message me if you have any questions.