Ecommerce sites are the new trend these days. Before you talk to a Web Development agency or start investing a huge amount onto this new online business idea, there are things you must consider.
First Thing You Should consider is the Budget.
Based on the budget, we can opt for custom made ecommerce solutions or ready made ecommerce platforms.There are a number of ecommerce platforms/engines available like Woocommerce, Opencart etc which can be used if you have a limited budget. But these Platforms are not a scalable if you have a longterm plan. But they would be the best way to give life to your idea in an affordable rate. Maybe you can give it a try and then if things go well then you can switch to a custom solution.
Second important thing to consider is the volume of Products
If you plan to open a online shop with 50 products, then the best way to get it done would be Woocommerce or Opencart. But if you plan to start with 1000s of products then you should go for a custom made ecommerce application itself. I dont have anything against readymade ecommerce solutions, but most of these free open source applications carry a lot of weight and are not optimized, it can affect the performance of the site especially when you have a lot of products. But this is not the case if its 50-100 product website.
Custom ecommerce solutions will obviously cost more than readymade platforms like Woocommerce, Opencart or Magento.
So the conclusion is that you should choose a solution based on your budget and the volume of the products you intend to sell. so the next questions is, what will I do if I have 1000s of products to sell and my budget is low? I would say please try to increase your budget :).