I do admin work for a very large isp in oklahoma and I can tell you ebay is NOT the way to go. The only thing I have ever seen them buy off of ebay are nic cards and old machines to use as firewalls.

check http://datacentertalk.com/forumdisplay.php?f=10

Sure you can buy off ebay, or you could buy direct from a supplier. Thing is, it depends on how you want to run your DC.
A) You could have a high performance DC with high performance/new equipment which would require you to buy from a supplier.
B) You could have a middle line DC with partially used and partially new equipment which would allow you to buy off ebay and direct from the supplier
C) You could have a lower end network and buy everything cheaply off ebay.

My suggestion to you, do none of the above. If your asking such simple questions you really shouldn't be considering building a DC. Its like someone purchasing an unmanaged server with cpanel and not knowing what ssh is.

I think hostaffect said it best but forgot to follow his own (as well as others) advice. "Come on guys don't be telling him shit...."