[Build Help] Can I trust sites like Craigslist, Kijiji, ebay and such?

As the title says, there are some good deals that I found on the sites. I'm interested in a power supply. According to the owner, it has been used for 1.5 years and I was wondering if it was worth the discount. It's relatively cheaper and I don't have to pay shipping, but I'm not sure if I should buy it because it's been used. Any help?