How much transparency for an online reputation system?
RobGR: Everyone should know the rating system.  People tend to ignore rating systems that are hidden or not easy to understand.  An example of a rating system that suffers from many hidden features is the "credit score" system; of course it is not designed to be useful to consumers, it is designed to be useful to the companies that know it's hidden factors.  An example of a scoring system that is defined, but generally hated for being opaque and complecated, is the BCS college bowl system.You should make it so it is not easy to game; the way to go about this, is to make it so that the easiest way to "game" the system, is to do something that you want to encourage anyway.   So instead of making a fake account and making fake transactions back and forth to up your score, give sellers a bonus for allowing new, unrated buyers to pay after receiving the item, for example.Think about how the standardized tests for private pilot licenses and etc work.  They publish thousands of example questions with the correct answers, and then for your test they select a few at random.  If you "game" the system by studying all the questions, you achieve their goal anyway, which is to learn the material as well as they can reasonably measure.Now, you could take out the reputation and knowing the seller part of it, but I think then you would have to step in and guarantee things for the buyer.I would do an ebay-like accumulative rating system, but have points be weighted with age, so that a good rating from a week ago is worth more than one from a year ago.  You might fade them linearly to zero, but I think your formula should give people who have had an account for a long time some weight just for that.I am experimenting with a scoring / rating system now.  it is for my site slackerfactor.com, which is mostly functional.  The site allows you to keep track of who paid which costs among roommates or a similar group; if someone is a slacker, and doesn't pay his fair share for a while, his "slackerfactor" score shows it, and the people who are doing his share similarly have a better score.I decided to make the score fade with age, linearly to 3 years, after three years it doesn't matter any more.  Since recent debts / credits matter more, someone who is up for re-evaluation as a housemate at the end of a lease or whatever, and has been slacking all year, has an incentive to clean up their act and pay a bunch of bills or buy groceries or whatever.  The main site doesn't display the slackerfactor yet, only the beta site, because the formula probably still needs to be tweaked according to how people will use it.Another approach you might want to put in your bag of tricks is per-user modifications of the score.  On Slashdot you can say you want to ignore "offtopic" moderations when deciding what comments to show, or to rank "troll" as a plus instead of a negative.  Similarly, you might offer users the ability to have some checkboxes such as "ignore rankings more than a year old" or "negative rankings count as double if the person making the negative ranking has a score above 50" and so on.  Giving people the ability to tweak knobs often quiets the most persistent complainers.