Is the marketsphere unclaimed property letter a scam?

No, the letter you received from MarketSphere Unclaimed Property is not a scam. It is legitimate. You (or the person on the letter) is owed money from one of MarketSphere's clients. MarketSphere is trying to locate the person on the letter to notify them that a check or other payment needs to be reissued from the client, or else the funds will be escheated to the state Treasurer's department to be held there until you come forward (or search the state databases) as the rightful owner of the funds.