The advantage of selling points by auction is that the supply remains limited to an amount I consider reasonable, no matter how many dollars I give out. So by supply and demand, the more dollars I give out, the less each one is worth. The mechanics of the auction are easy. On the bulletin board I post a sheet. Each line of the sheet records the students bids on one extra credit point.
Students tend to hoard their wealth until time for grades, which is of course a bad strategy.
The following Monday, I call students for their money and record their points. Once the basic idea is in place, math dollars become useful for many things. One of the first things I thought of was to improve the quality of the document itself. In their current incarnation, each of my math dollars shows the face of a mathematician on one side and has a short description of the mathematician s life on the other. This way I m imparting a little math history, without actually taking class time for it.