I used to have great success with selling candy bars. I'd put up a sign in the employee kitchen, and then leave the box open (with the envelope of change) in my office. I made a ton--I could have a box going all year long, basically (after the first one, a coworker said, "Can't you just go to Costco/BJs and get a bunch of candy? You can sell it at a markup and give the money to the school. Or heck, keep it!"
And even when my office was right next to the big, open area, and I left it open after hours, w/ the candy very visible, I never had anybody steal candy or money. Some people would even stick a little extra in, so I was always ahead.
It was great, bcs it was actually cheaper than the newsstand in the lobby, and besides, people didn't have to get in the elevator.
And I didn't have to push; I just put up the sign. Also, I felt OK with the lower-earning coworkers' participating. I never felt comfortable asking them to buy stuff (never asked directly; just put up a sign).