If you can gather evidence showing that these individuals or couples specifically misrepresent themselves as adopters, and are "flipping" multiple pets on a regular basis, I would file a complaint with Craigslist. Craigslist has their own rules defining what is illegitimate, and this might violate those standards. It doesn't have to be illegal to get their posts removed.
Also, this sounds like good advice for "posts to watch out for" in general.
The page
http://www.cavyspirit.com/surrendering.htm has a link to "[FONT=arial, Arial, Helvetica][FONT=Verdana][SIZE=2]
How to Find a Home for your Guinea Pig", but the link is dead. It would be really good to get this info onto a permanent advice page or a sticky thread, such as the "About Rehoming" thread in this sub-forum.
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