It's fine to share possessions, food, clothing, etc. freely and without reservation, because no one is going to be graded on eating the casserole or wearing the donated jeans. Patient safety won't be riding on the box of dishes you gave a friend. There will not be an exam for the toys you passed on to a neighbor. Sick and vulnerable people will not be shortchanged by your generosity. Learning will not suffer nor will good study habits be compromised if you are motivated to help others.
The same cannot be said for schoolwork.
Although it may seem like you're being kind by giving away your materials freely, ultimately, you are not. I'm not talking about helping a classmate in a bind now and then. But spoon-feeding those who have poor study habits (other than helping them to learn better ones) or enabling people who just want to coast through the class is about as responsible as letting someone copy off your test paper.
Some have protested that it doesn't cost anything to be "nice." I disagree. If you gift-wrap a superficial grasp of the material (just enough, say, to pass a test) you give the message that it's okay to game the system. Undisciplined takers (as opposed to genuine study group participants) learn that ethics don't really matter. The end justifies the means. Besides robbing classmates of the chance to internalize the subject at hand, you may also short-circuit a much-needed reality check. Imagine the difference in outcome between a person who found a way to skate semester after semester and one who, with the clarion call of a failed exam or a flunked class, realized this isn't high school anymore.
Add all of this to the concerns some have stated about plagiarism, and it doesn't appear to be a good idea to post or share certain items. In many (if not most) academic honor codes, the one who assists the cheater is viewed to be as culpable as the one who does the cheating. I imagine this would only intensify if money had changed hands, an arrangement that bears an uncomfortable resemblance to buying and selling term papers on line. It boils down to doing someone else's work for them. That isn't fair to anyone in the group, and it could expose many people to extra risk down the road.
Just to clear up any confusion, I think we're all pretty much in agreement that there is a big difference between a generally responsible person taking part in a study group or needing help through a bad patch and a loafer who just wants someone else to do his or her heavy lifting.
Sometimes being nice means saying no.