I just took a midterm on this material and one of the questions asked what the argument in support of epistemic relativism was. Well, unfortunately I forgot to finish what I had started here, so my essay wasn't as good as it could have been had I kept this thread going. At least it is never too late to continue.
Anyway, according to Paul Boghossian in the book Fear of Knowledge
1. If there are absolute epistemic facts about what justifies what, then it ought to be possible to arrive at justified beliefs about them.
2. It is not possible to arrive at justified beliefs about what absolute epistemic facts there are.
3. There are not absolute epistemic facts.
4. If there are no absolute epistemic facts then epistemic relativism is true.
5. Epistemic relativism is true.
The argument is certainly valid but there is a serious issue with the second premise. If it is not possible to come to justified beliefs of absolute epistemic facts, then that would suggest that it is impossible to come to the conclusion that epistemic relativism is true. This would be an example of formulating a justified belief about an absolute epistemic fact--the nature of the very epistemic system argued for.
The problem comes when two opposed systems come into contact with one another. One system would have to justify itself over the other. Obviously one party would believe that the other parties system is false and their's is correct. How can one system be justified over the other within a system that does not allow for justified verdicts?