Just be prepared with URLs and proof that he is selling your stuff and that you want to press charges for theft.
Except, from the cop's point of view, it *isn't* theft - literally and legally, when they file charges, "Theft" will not be mentioned.
The local cops are not typically prepared to deal with copyright infringement, or the computer forensics involved. We are talking about a cybercrime, which is kind of out of their baliwick. Moreover, if he lives in a different state from you, or from the place where you agreed to sell it, then it becomes an inter-state commerce issue, a Federal matter (and copyright is a federal statute, anyway), and the local cops will point you at at least the State Police, or more likely the FBI.
Now, the FBI are *entirely* equipped to handle the matter. They have the jurisdiction and the expertise. But copyright violation on some sales of one publisher's RPG books are probably small potatoes compared to the other cases in their load, so low in priority. This is why class action has more pull - you're now talking about tens of victims, and they start taking notice.
He shouldn't need to be sued. Copyright law is a criminal affair, not a civil one.
Well, you can do both. In fact, if you want to get any money back, you will probably need to launch a civil suit.