Too many pages to go through and many opposing points of view.
My position and belief is that you should keep the phone. However, that and $5 may get you a cup of coffee at Starbucks. The reality of the situation becomes a civil matter. The holder of the phone can and will be held liable for any damages received by Best Buy from VZW. Whether he purchased the phone or found it in a bar (think Apple's phone), he is in possession of a product that is not released, contains confidential information, and protected under numerous laws for VZW.
At the end of the day, Best Buy could seize the phone if they wanted to. It would have taken a lot of legal maneuvering to do so. I am actually surprised Counsel for BB did not contact you directly. Best Buy can still hold you civilly liable for fines levied by VZW. VZW can also hold you liable for possession of illegally acquired merchandise (trust me, I was sued by Addidas over a very similar issue). Right, wrong, or indifferent, you can be sued by Best Buy and VZW. All the civil suit needs to do is prove that Best Buy made every effort to re-acquire the phone (although I don't believe they truly did). The cost of your own counsel and time is not worth it. The worst part is the OP is still on the hook for a potential civil suit. If Best Buy's liability is significant, they will go after you.
You can argue that BB should not have sold the phone, it was the employee's fault, or any other argument. What it comes down to are the facts. The facts are the OP acquired a product that was not to be released, the company made the effort to get it back, and it was not recovered. Those are the facts. Everything else is personal opinion and "screw the man" mentality.
The cost of 1 hour with an attorney is going to be over $300. This is out of your pocket and no chance at recovering it. Drag it on with the BB attorneys, it is more money out of your pocket for yours. BB's attorneys are getting paid whether they work or not. Might as well stick it to someone they have a chance to win against.