You should piggyback your plugin into popular installers to get this on millions of computers. Then, don't make them push a button to post it on krrb, do it automatically. Make sure your TOS at install time says this is going to happen. (I may or may not have been involved with a company that did this to great success.)
So instead of providing a useful service, they should distribute malware designed to... what, maximize their chances of being sued by craigslist?
The plugin today quite arguably scrapes Craigslist programmatically. I think Craigslist has a case there.
However, if Krrb instead injected JS at post-time to publish cross-site, I think that argument becomes much less convincing.
You can make a case on either side, but I think doing it at post time is important (for one thing, it guarantees that the publisher is actually the person krrbing it)