Obviously, we have no way of knowing what the person who bid $78 for a 100 mile certificate was thinking. But maybe he didn't quite loose his mind. Maybe the person is not familiar with FF programs and thought that it entitles you to a 100 mile flight, or 100 miles of a flight (not that you would get off in the middle- rather, you use a number of cert's for one flight.)
In Canada and UK and maybe somewhere else there is a (lousy) program called "Air Miles" in which the number of "miles" needed is supposed to be the number miles travelled round trip. (The problem is that they think you fly about 1000 miles YYZ-LGA-YYZ!) Maybe this person thought that AAdvantage works in a similar way. Ignorant but not quite stupid.