The best method is to use a professional Craigslist posting service that uses an approved method for creating ads in multiple cities. While “unauthorized posting agents/services are prohibited,” you can use ad software that uses the official Craigslist API to post nationwide.
If you do use an unapproved Craigslist service or DIY, a proxy will likely be necessary to hide their IP address. This is a way side-step the rules of Craigslist at the risk of having your account banned or posts blocked.
The biggest challenge will be avoiding the Craigslist algorithm for detecting spam if you’re posting the same item in multiple cities. That essentially means that each ad will need to be unique content. The content doesn’t need to be entirely different between ads but most likely rewritten for each one so the sentences don’t match up. Duplicate content checkers, like DupliChecker and Copyscape, can come in handy for testing your content.
It can also help to post ads from different email accounts for each location. Craigslist tracks information and behavior on each account so it helps to spread out the distribution to specific accounts.
Ads might still get marked as spam by Craigslist since we don’t know exactly how their algorithm works. Then a software platform would be the best option.