Flippa is the largest marketplace for smaller sites and domain names.
Verify their traffic (via Google Analytics) and revenue (via profit/loss statements). Check for "toxic" backlinks that might get the site penalized by search engines.
How to buy a domain name | Domain purchasing guide - Cloudflare
This is often called "website flipping" or digital real estate. You are buying the traffic, content, and revenue stream of an already active site.