If I'm targeting a very low competitive long tail keyword and posting on craigslistm what are the chances it will show up on the first page of google? I've done this in the past with success, but lately, it doesn't seem to work. I post a great long tail in my craigslist ad and I can never find it on google. Any thoughts?
Maybe your CL posting has expired, or the Google engine didn't crawl it at that time. You can heighten your chances if you also submit the long tail keyword in multiple ways (blog comments, signature links, backlinks) This way Google has a higher chance of finding your CL post.
You might try backpages as well, which is similar to Craigslist. I do not get a lot of traffic from them but they have decent pagerank (PR 6 I think) and as I recall they are a do follow link.
Craiglist is a very strict classified ads site. Better be careful on how you'll use that site for your SEO.