This one is a bit tricky. While you are not doing anything unethical, there could be complications depending on the state and how much is known to the inspector in case of that. For example, the state of AZ would not allow you to just "use" the same VIN on a new construction frame. They would require you to come in for an insection and issue a reassigned VIN for the new frame. If it was found that your frame has been restamped with a VIN associated with a previously manufactured factory frame, it would be subject to impound. Will it ever happen? Maybe or maybe not but it does leave a "story" behind with the vehicle though that you would either have to choose to disclose or not. Then you have to decide if you don't disclose it and there is a claim later due to it being sold in a state that is tough, It could be your liability. At the end of the day, many will just tell you to do it and not worry about it. But, it is not there problem either.