Multi-sites is a Drupal feature that makes managing many websites easier. The idea with multi-sites is that you can use a single code base for more than one website. When you update that single code base, you update all of your Drupal sites. Each site can still have its own theme, modules and content.

With multi-sites you can have many sites with one database, or many sites with different databases.  In this tutorial we will chose to have multiple sites, each with their own database.

This is particularly useful for managing the code since each upgrade only needs to be done once. Each site will have its own database and its own configuration settings, so each site will have its own content, settings, enabled modules, and enabled theme. However, the sites are sharing a code base and web document root, so there may be security concerns with multiple administrators.

But Marketplace is set of modules that uses Drupal Commerce and let you create a marketplace. This marketplace has some features like Stores,Store access control,Marketplace orders,Store orders,Shipping fees and Payment to stores. This features only helps you to manage eCommerce functionality like handling orders, handling stores, shipping, payment etc.

But this module is not actually not helping to create or manage any separate website with single code base or database like Multi-site. Rather it provides a new entity type for stores, a new order type for marketplace order, a separate module for managing payment etc. So for doing this functionality it is definitely using separate database and coding structure.

So if anybody wants to create a new website but with single Drupal installation(including core code, contributed modules, and themes) it is best to use Multisite in Drupal.