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.
If you have an idea and need a site built from scratch, you aren't buying an existing "business"—you are buying a service. how can i buy a website
How to buy a domain name | Domain purchasing guide - Cloudflare Verify their traffic (via Google Analytics) and revenue