The county thought you paid too little for your house. They are probably trying to stop people who buy homes from owners who sell at below market price, and then get another cash payment under the table, or relatives selling to each other.
IMO, the County became aware, probably many years ago, of the material volume of ethically questionable transactions that were occurring in the market and developed this probably verbal-only policy to deal with them to limit the doubloon losses from the County treasure chest. A policy I would also consider of questionable ethical origin. Since they do lack the resources to investigate every transaction to a level of detail that would inform them, in fact, of an actual and not just questionable unethical deal, they simply blame every rooster they come across for making the sun rise and sit back to see what the rooster does in response.
