Did the seller give you the ESN to check it out with Verizon. EBay is where people sell there phones when they owe money to Verizon, if the seller owes them money or the phone has a bad ESN for some other reason, Verizon will not activate it. Good friend of mine got burnt, filed a complaint with both eBay and PayPal. Both said he was correct and the seller owed him a refund, but he was never able to collect.
If the sim cards are the same size you can just switch them and the phone should activate on its own. If they are different sizes you'll have to run to a corporate store and pick up a new sim.
Hope all works out, I think your gonna like the gnex over the Bionic.
Sent from my Illudium PU - 36 Explosive Space Modulator.