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In most cases, postal workers know where all of the vacant houses are. When a house is vacant, the post office stops sending mail to that address so the mail doesn’t pile up. When they do this, they put a vacant slip in the mailbox of that property.

You can contact local postal workers in the neighborhoods you are interested in buying in and ask them if they can provide you with a list of all vacant houses on their route. You can offer to pay them $5 a lead or work out a way to pay them for their time.