As an SMB, I buy secondhand whenever I can, but the key is supportability. I learned the hard way one weekend when a Dell server went down in an odd way (CPU machine check bug associated with power state control) and the seller, a very large Dell aftermarket reseller on eBay, had no weekend support. I desperately wanted to call Dell, but couldn't.
That feeling you get when you're completely on your own, trying to get a production machine back in service on a deadline, well, it's just no fun. So buy used, yes, but be sure you can get the support you need, when you need it.