Ecommerce

Best solution is probably to go with the instructions at: https://www.drupal.org/project/commerce_demo
composer require drupal/libraries bower-asset/jquery-simple-color bower-asset/spectrum
Install site.

How to build drupal commerce on top of drupal 8 with demo content (https://www.centarro.io/blog/how-build-drupal-commerce-development-envi…):

Prestashop together with Thirty Bees is a PHP based webshop system widely used by many webshop owners.

Modules

Multi store and marketplace: https://www.knowband.com/prestashop-free-marketplace-modules

As a drupal fanboy I like Drupal Commerce, but there are many others out there.

Drupal Commerce

https://github.com/drupalcommerce/demo-project

Default theme: https://www.drupal.org/project/belgrade

Read more about how to install Drupal Commerce.

Prestashop

Woocommerce

Wordpress commerce.

Magento

2017

2017-07: Top 10 Free Drupal Themes (vardot.com). 

2015

Be inspired by this search on Drupal.org.