Building an e-commerce website is a pretty complicated process.
Not only do you have to know the back-end, the database, and making it secure you also have to contend with dealing with sales tax, shopping carts, orders management, inventory, and etc…
You’re much better off going with Shopify and letting them handling that complicated stuff for you. From there, you can focus on just theme development using your html and css skills.
Magneto is another good ecommerce platform most enterprises user too.
Something else to consider is that business owners aren’t looking for someone who can just build their site. Most will build the site on their own using the website builders that Shopify, Wordpress, or any other CMS offers. So, you have to learn extra skills to give yourself an edge. For instances, having experience working with google analytics and using SEO will make you more valuable. Or being able to design a plugin to help drive sales will get you picked out of a sea of newbies.
I don’t have the option of using Shopify (edit: or any other platform which requires credit card, debit card payment). It doesn’t work in my country. I am almost pioneering the introduction of e-commerce to where I live.
What about Wordpress with WooCommerce? Squarespace has a decent platform with ecommerce plugins.
I can’t pay for their services that’s why