Theres a scam similar to this, where people will tell you that they will buy the domain from you only if you purchase Sedo's appraisal service. After you pay Sedo whatever it is to get your domain appraised, the scammer says they are no longer interested. The idea is that you will get a refund for your appraisal service off Sedo, and somehow they are trying to put Sedo out of business.
If you google 'sedo appraisal scam' you can get some more info on it.
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Timwah (I'm at MSN .com!)