The answer is very simple! Make the post as Unique as possible!

Sometimes the things you should change are not the IP, Email or Content but the way you are thinking!! Marketer has his own characteristics and its way to thinking! If craigslist can use this psychological approach to catch posters’ thoughts then program these to their software, your ads will be blocked for ever! And you can’t solve it by yourself! You should look for professional to help – a third party view point.

Are your ads flagged? How to solve it?
Flagging is another frustrated thing for lots of advertisers! People flag each other’s ads to maintain their ads position on craigslist. Many of them even use software to flag other people’s ads! Even some software can completely block one person to post! This will further frustrate people to post on craigslist! As frequent posters and loyal audience, many people lost confidence to post on craigslist because they are facing hard time from both Craigslist auto spam filters and competitors’ flagging machines! How flagging system works? It relates two elements:
1. Your IP address
2. Key words in your content

How to solve it? It is very easy and simple – Make Your Ads More Unique!
I know you will say this is B.S but indeed it is the truth! As professional Craigslist Ghosting and Flagging Trouble Shooting Expert, this is the best solution I use to solve many many cases!