It was one of the top real estate portals in the world. A lot of geolocation searches. New search every time someone moves the map. A ton of data sent to the client. Analytics in every page view.
No clue how a shopping cart or checkout flow would drastically increase database load. It should just be basic CRUD. Building a shopping cart is something every student makes. Pages in a web store can be cached relatively easily since items won't change often.
A primary DB with a few replicas and caching can go a really long way.