Blah blah "price discovery is a service in a free market blah blah" does not apply to Craigslist.
There are a few ways a job like this could be ethical and productive, respecting the hippie community culture of Craigslist but improving efficiency in exchange for a share of the profit, instead of merely siphoning gains out of the system:
* Buy items that fail to sell (perhaps because they are geographically remote), and resell them from a more accessible location. http://craigstruck.com is a variation on this model.
* Buy junk, fix it up, resell it. ("Upcycle"/ "speculative repair work"
* Take good stuff that doesn't sell, and re-market it better.
[0] "spec" is a funny auto-antonym: "Speculative" or "to specifictaion"