do you have a link to the old ad? then you could try http://www.cachedpages.com/
^If you can't get the cached craigslist page, you can recreate it yourself using the firebug add-on for firefox and then take a screenshot, just make sure the date and time on your desktop matches up.
How do I do this? If you could explain I would REALLY appreciate it!!
This is for informational purposes only. There's most likely better ways, but this is easy enough for noobs. Fuck, I'm not even a script kiddie really.
Firebug allows someone to make a webpage say whatever they want, but if they close the window, they lose it. Someone could install
firebug and find a current CL ad in the same category. All they'd have to do would be to open firebug, run their mouse over the lines of code, and expand them until they found what they wanted to edit, then close firebug and take a screenshot when they've finished.
To cover their tracks, they'd want to photoshop the firebug icon out of the toolbar on the off chance it gets recognized (a screenshot of firefox before installation would be useful), change the numbers in the URL so that it produces an error if someone were to type it in, and change the date/time on their browser to match what they made those on the page (right click on the time -> adjust date/time. In windows at least). As backup, they could even disable their wifi, take their laptop to court with them and bring up the page "right there" in front of the judge.