There are a few other places that people who aren't affiliated with the club might post up things for sale, just in case.The UNDerground
UND's own classifieds system.Craigslist
There will be a deal in Grand Forks every once in a while, other places to check might be Fargo and Bismarck (to say nothing of the Twin Cities).Campus Dakota Classifieds
The whole website should be upgraded soon, so the classifieds system should get more use. Covers North and South Dakota colleges.
General tips about buying things directly from other people include the usual about not getting scammed, making sure you get what you paid for, and making sure you're actually getting a good deal. More bicycle-specific tips would be useful to have here--does anyone have any to share?