nopCommerce is a fantastic open-source, stable, and super extensible eCommerce platform based on cross-platform .NET Core! It's perfect for any size business.
nopCommerce has an enterprise-level architecture - a very strong example of what a .NET developer should be looking at the front-end, business logic, mid-tier, and database level.
Absolutely free. 60,000+ of successful clients. The most popular ASP.NET shopping cart in the world.