Craigslist isn't the best for servers, but for some of the very cheap stuff it might be okay. Ebay is good too but usually just takes a while to sell. There's a section here for hardware sales - http://www.webhostingtalk.com/forumdisplay.php?f=163. Probably just as good as any other outlet.
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We were in that same position back in 2008, with several big stacks of P4 big-brand name (not whitebox, not Supermicro) 1U rackmount servers that had been retired after a major hardware upgrade cycle. All were in like-new shape and only 2-3 years old, two HDs each, 2-4 GB of RAM, etc. We just didn't need them anymore and were offering them really cheap (think $125-150 depending on config).
Advertising on WHT didn't work at all. Asset recovery companies wouldn't even respond to our inquiries. Selling on eBay was just to slow. It would have taken forever to get rid of them all one-by-one that way.
We eventually sold them to a local dedicated server provider who offered them as while-they-last bargain-basement deals. It's my understanding they sold out pretty fast, so they recovered their investment within probably 60 days. Win-win for both of us.
eBay is a good place. They will give you good support when you're going to sell something. You won't get sales fast in beginning. You have to increase positive feedback from buyers to get sales fast in eBay.
Be prepared to deal with the occasional idiot on eBay.
In a personal capacity (not as Thor) I sold a server on ebay a year or so ago. The guy who bought it had no idea what to do with it. I spent a fair amount of time trying to help him, eventually offering him a refund for the sake of a quiet life. He returned it dented and scratched and then left negative feedback.
Unless you've got the time to deal with stuff like this, I'd be tempted to contact some of the bulk server sellers on ebay who might take the lot off your hands. Or else put them up as one big lot.
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