How does Krrb verify this? How does Krrb ensure that it's the post's author, and not some random bot hitting executing their javascript in an effort to scam users?
> Craigslist owns the copyright to them as soon as I submit the ad and thus I cannot post that content elsewhere?
You absolutely have that right. It's your data, you can re-post it wherever you like. However, Krrb does not have the right to scrape data from Craigslist, on your say-so.
The content was posted to Craigslist with a license for Craigslist to publish it, not for Krrb. Much like a book author gives up certain rights to distribute a book through a publisher (including where and when it's published), the user is giving up certain rights to post the content on Craigslist. Whether this is a bad thing or not is up to the author of the content, not a 3rd party to decide.