Spree Commerce is such an open-source e-Commerce solution.
We’ve built a marketplace hosting over 3 million SKUs based on heavily customized and optimized Spree Commerce and a farm of 15+ dedicated servers (load balancers, cache servers, http servers, app servers, database servers and so on). It handled over 15.000 requests per minute in external load tests / DDoS simulation.
We’ll be happy to share our know how.