Craigslist is becoming harder and harder to post because they keep changing up their rules n such. I use craigslist for a small source of income usually about 100 a day, but if you know what you are doing you can really scale it out. A really really good course to pick up on craigslist is craigslistblackhat. That course will teach you how to get your ads through. I can usually post about 500 ads a day and get around 400 through, which isn't too bad. I highly recommend that course though. As for other sites... Backpage has been treating me fairly well lately. Gumtree is getting to be like craigslist so its not very fun, but still works on occasion.
If you used the same/similar title or same/similar content, your ads can be easily identified (auto flagged) and then promptly deleted. Also, they probably noticed high posting activity for a given IP.
What kind of ads do people post on these sites? Do you give a landing page link to an affiliate product or just an advert to check out your website?
I've seen hundreds of them being an advert to check out a website, especially electronics (mobile phones, game pods) and "typing/ad posting jobs at home" listings.
some are as following: http://www.usfreeads.com/ http://www.locanto.com/ http://eesycheesy.com/ http://www.backpage.com/ http://tapthat.co.uk/ http://www.stumblehere.com/ http://www.webcosmo.com/
I launched an SEO career by placing an ad and linking it to my website... Very basic, I know... But the first time I did this I got a call and converted that call into a client. I was hooked on learning more... And I did! Gotta love Craigslist.