Instead of writing a backend, doing a login screen, setting up a database and all the other things with a backend, for the time being I've just done the front-end javascript to create an ad, and then tell the person to copy/past the ad that they just created and email me. After that, I've got a few scripts locally that will publish their ad.
This workflow will allow me to iterate quickly, without wasting so much time and effort on things that in the end doesn't matter.
Following @pg's "Do things that don't scale" approach, I've streamlined my workflow to run the site manually without building the backend.
Feedback welcome.