"There's a sucker born every minute." (quote possibly from P.T. Barnum)
The counter to this is don't believe everything you read or are told (whether on the internet or elsewhere). While I believe that most people are generally honest and trustworthy, there are many unethical people who are eager to steal from you.
Online marketplaces & exchanges (Craigslist, eBay, freecycling lists, online classifieds, email lists. etc) are great enablers of more efficient distribution of goods, but be warned -- they can be misused as well. The following article from PC World about Craigslist scams applies to all electronic venues. Please read and be careful out there!
And for freecyclers see in particular:
A similar scam occurred on a San Jose freecycling list in 2004/2005.