Hi there,
I think I'm the man for your job.
I coded 3-4 years ago a classifieds site for my personal use so I know the drill, but I evolved in my coding skills since then so I could produce a much better version now.
I am only using core PHP coding (from scratch) on all my projects, unless the client specifically asks for a php framework (I've been using Codeigniter as framework).
Anyway, for the given budget (bid) I could build your classifieds ads site from scratch, using twitter bootstrap as css boilerplate (to make it responsive) in 10 days at most. We'll build a simple and flat layout site, fast, low on memory and cpu and responsive. Leaving room for further styling and visual improvement.
I'll build both front end and a back end where an admin can manage ads and users (eventually).
Message me if interested, I would really like to take on this one.
Regards,
Calin.