Buying a used phone can be a great way to get a great deal. It can also be a way to get scammed out of money with buying a blacklisted phone. VZW has given us the way to test and see if the phone you want to buy or get can be used on the network. Usually you would have to call them and wait to see if they can run an ESN check which can take some time or you would have to go to a store and check.
You can check your ESN by going here, entering the ESN and you can find out for sure – http://www.verizonwireless.com/b2c/estore/certifieddevice/cd