Most of the time you will barter your tickets to close to what "face value" is listed on the tickets.  However there will be certain situations where you will be able to barter your tickets for what is the "fair market value" which can be much more than the "face value" listed on the tickets.  When you find a person who absolutely can't attend the event they have tickets for, you will the a greater chance of getting them for less than face value.

One of the best place to place a barter ad for your tickets is on CraigsList.com in the Barter section.