Below are the links to the source code for the E programs, marketplace.e and marketViewer.e. Together these comprise the marketplace system demonstrated by Mark Miller at FC'00.
The first person to find a security breach in the system will receive a prize of one gram of e-gold and an autographed copy of EarthWeb. The finder of a second, different security breach will receive an autographed copy of EarthWeb. The arbitrator in case of dispute is Mark Miller (the Mark Miller who owns and operates www.erights.org).
For a true security breach, good targets include the SoleBookKeeper and the CapabilityIDs for the individual participants in the market. If, using a market viewer of your own design, you are able to extract from the marketplace a capability on the book keeper, you will be able to transfer assets at will. If you find a way to retrieve another person's CapabilityID, you will be able to impersonate that individual.
Source for the marketplace.
Source for the viewer.