And, let's not forget, there's fiction in a lot of those products too. You have numerous writers with their name on the book. It might even be an issue that someone's poem or whatnot in a book possibly could have copyright problems. Not as likely to be sure. But, still possible.
And, again, how much is it going to cost you to track that down? People today are a lot more informed about copyright issues thanks to all the press coverage of things like the RIAA. People know that they have rights. They might not care, or they might. As a business, you can't put something out and hope that no one complains. All it would take is one or two claims, even spurious ones, to wipe out profits, or at least to set profits way back.
Just as a thought, and I honestly don't know, but, who owns the character rights? Who owns, "Lareth the Beautiful" as a copyrightable idea? Or whatever proper noun you might have tossed into some sourcebook. You'd have to go back to the original contracts, which are now decades old, buried in some store room somewhere, if they exist at all.
I have no idea if this is an actual issue or not. Maybe it's nothing. IANAL, so, it's way above my pay grade.