Any new visitor to [Nigeria] is bound to notice the odd phenomenon that literally thousands of houses and buildings in cities and towns bear the message "THIS HOUSE IS NOT FOR SALE," painted prominently near the front door. Ask any Nigerian the purpose of the message and they will quickly tell you that it is to prevent 419 [scamming]. Apparently, one popular method of 419 is to assume the identity of a real estate agent or simply a property owner trying to sell one’s house. In Nigeria’s cities and towns, where the real estate market is tight, buyers can be induced to make down payments to secure a later purchase, and in some cases entire transactions have been completed before the buyer discovered that the deal was a scam.
That is from Daniel Jordan Smith’s informative and entertaining A Culture of Corruption: Everyday Deception and Popular Discontent in Nigeria. Here is the book’s home page.