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Do a search on 'Karma' and you will find it is one of the features of a forum module and is the closest thing i can think of to a way of rating a 'reputation'...got no idea how it works though.
Personally I am not into this sort of thing as I find it 'judgemental' - My experiences on Ebay have taught me that feedback systems are really dependent on the individual standards of the person making the feedback and many of them are unforgiving and perfectionists - as a result somtimes their feedback is very offputting and that is saying it nicely.
I much prefer our system of users 'just popping in' or 'can't stay away' or 'friend of xoops' - as far as 'Reputation' goes you get it by what you do not by what you are rated at...Your number of posts, your kinds of posts and the wonderful thankyous after some effort to help are enough to establish 'reputation' around here IMHO.