Hi there,
Understood the requirements. I imagine the homepage would be the interactive map of US, with each country being clickable. When the country is selected, show a list of towns in that country. When the town is selected show the bulletin board, and if there are free slots allow users to place their ad there.
Ad could be an image or just text. All ads should have keywords, title and description, and probably an expiry date so that slots get freed up at some point.
Ads on the bulletin board would be clickable, taking to ad details page. All this with SEO in mind, for example if the ad was "Plumbing services", and it was listed in Phoenix, Arizona, then we would have a page title "Phoenix Arizona Plumbing services" with all the ad keywords and description, making for a good SEO.
Ads are shown once they are paid for using PayPal, until their expiration date.
I never worked with Apple login, and since the budget is tight I can't promise to do that, but I would allow for registration using an email, Facebook or Google account.
Let me know if I left out any required features.
Regards,
Julijan