You certainly can purchase a Surfbeam 2 modem on eBay, but unless you are an authorized installer, I'm not sure what you can do with it. The modem has to be initialized with a Modem Key and provisioned. The modem serial number is reported upstream to Viasat. I'm sure there are ways to get around this, but not legally and not advisable.
JEP is correct. If you purchased one off the internet to use, there's no guarantee it would even work with our service. The previous owner may have dropped it or something during their time of use causing it to be sup-par. You really want a new one to have the best possible equipment.