What is the most efficient way to create and manage 500 sales/landing pages
Disclaimer: I'm a maintainer of Aegir.

Aegir is inherently multi-site, you can have many sites on a shared platform, which means you can have many independent sites with the same codebase. This is a lot more efficient resources-wise than having multiple sites without a shared codebase.

If you often need to create new domains, I developed a Software-as-a-Service suite for Aegir: https://www.drupal.org/project/hosting_saas

Since there is E-Commerce involved, you'll want to have an SSL certificate, with Aegir you can easily use the same wildcard certificate for all the sites if necessary.

If the landing pages don't need to be customized (much), you can even have a single site with 500 aliases in Aegir.

Lastly, one of the main advantages with Aegir is that you can simply clone a site when you need a new instance (which is what we do in Hosting SaaS).