Usually when a patron calls asking what the library accepts for donations, I make a point to say that National Geographics are not accepted. A lot of times, patrons will just bring them in by the boxes and by then the patron has taken the time to drive to the library and bring them in, so none of the staff turn them away.
Some really think that their "collection" has some real monetary value: "Some of these are really old!" or "This is a complete collection of the 70's." I'm not one to burst that kind of bubble so it's all just smiles and nods and an occasional, "niiiice."