I'm a website builder, I start everything from scratch, and I can confirm the same thing ju112 says.
Buyers become interested after they do content analysis. If my site gets traffic on pages for terms they rank well for it's those rankings and that traffic a buyer wants. Lots of times they don't even care how much the site makes if the price is low enough. There are a lot of services out there where you can see how a site is doing in search.
Next the buyer wants to get an idea if they can get even more traffic by doing something I might not have. Maybe they are better at SEO, maybe they are better at promotion, maybe they are better at monetization, it's not really something you can verify but buyers want to know they can buy the site - fix a few things and make even more.
MOST of the time the sale price will be a multiple of monthly earnings but I have sold some sites I hadn't monetized yet.
Tip: ask questions! A legit seller with nothing to hide won't mind answering as best they can.
- When the seller claims lots of earnings but you can't verify traffic it's a red flag.
- When the seller doesn't claim much earnings but you can see the site gets decent traffic that's a good thing.