My experience is a few decades old but it was pretty simple. Some servers for the text and some for the images. The data is super cacheable, the hot set is really hot (current listings), and storing text and some image pointers is pretty simple for even a moderate database (which can be split by metro).

I was on the security team for eBay/PayPal at the time they took a minority stake in Craigslist, and one of the jobs we got was securing their infrastructure (they didn't have a security team).

I wonder if they still have that arrangement with eBay...