HSLD and other admin,
UND established a site for students to sell things on. It works pretty nice, no spam, if you want to check out what someone has you just go to the site. You could make another section of the site accessible from the main page that members could log into to view things that other members have for sale. Take a look at what UND has made. Obviously there could be improvements but it is somewhere to start.
http://www2.und.edu/org/underground/