I have been through this many times.
Craigslist will "softblock" accounts based on various information. This could be dependant on how much you post, how frequent, how many flags you get, posting commercial in non commercial sections, etc.
Once your account falls below an adequate user threshold you will be softblocked.
Softblocking will automatically "flag" ads making the user think its somebody else doing it, when it is actually craigslist itself.
The only way to fix this is to change your IP address, clear cookies, create a new account, and use a new phone number & email address to verify.
If you have cookies stored from your old IP address, use an old phone number or email in the ad, or anything that can be linked to your old softblocked account...the new account will be softblocked instantly.
You can read up on all it here.
http://www.craigsflagger.net/