It may have something to do with your DNS (Domain Name Server).

If you're running Windows, open a Command dialog box by Clicking Start->Run->cmd (XP) or command (98/Me).

type

tracert www.ebay.com

You should see a bunch of lines of information that tells you ping times corresponding to each hop between you and eBay. If you see some hops without corresponding names or ip addresses, it means you cannot reach beyond the last resolved address. Furthermore, if your DNS is not working, I suggest you talk to your cable provider. Most likely, it's their fault.