If anyone is seriously looking to buy a project, they care how much revenue it currently generates. Even if that number is zero, and you are just buying the code, buyers need to know where things currently stand.
Yeah, 30k for a side project is a lot of money if literally the only thing on the site is two screenshots.
I’d even call it laughable.
The site is a listing site, it's not like you click a buy button and then you all of a sudden own a project.
Of course not, but I’ve seen similar concepts before. And they all list some information about the business (especially monthly income) that are very relevant.
To add an example, flippa.com is a similar marketplace and it displays financials and even integrated Google Analytics data from websites for sale.
And I would add some idea of usage. If the project has no revenue but some traffic / traction that is much different than code running on a development server.