Grav can do most of the basics for a classified site:
categories
content with images
user accounts w/registration with login plugin
ability to search with simplesearch plugin
ad forms with form plugin
However, there is still some work left to do. It would require a plugin that lets a user create and manage ads. These ads would need to be saved as actual pages or in some format that could be injected into actual pages.
It’s not totally trivial but definitely doable via a custom plugin. You would probably also want an admin backend so you can manage these posts when needed.
If you would like professional help with such a project, our development arm. Trilby Media can help you out.