Nathan,
Rule #1: Always find your buyer(s) or markets BEFORE cutting your trees. DO NOT cut your trees first, then find a buyer...BAD, BAD idea :-(
I wouldn't worry about metal in the trees if they were coming from a forested setting. If your wooded "lot" is more of a residential lot than a house tucked into the woods, then yes, examining the trees for metal may prove beneficial.
If you have the means to transport your logs – great . However, if you truly have veneer black walnut logs, then simply have a professional consulting forester help you market the trees or at least appraise them for you. With the red-hot walnut prices right now, you could easily get 3-5 bids for your trees – assuming they really are veneer quality. By the way, most good, reputable veneer buyers will pay shipping freight for your delivered veneer logs.
Hope this helped!