The below is listed on the Idaho Residence Instructions for Sales Tax on purchases of vehicles, boats etc.
Sales tax is due when you buy a motor vehicle to use in Idaho. For Idaho sales and use tax purposes, a motor vehicle is defined as a car, bus, truck, motor home, or on-highway motorcycle that's required to be registered for use on public roads. Vehicles such as snowmobiles, boats, ATVs, UTVs, and motorcycles intended for off-road use aren't considered motor vehicles and not all exemptions from sales and use tax apply.
If an Idaho resident buys a vehicle outside of the state, use tax is due (with credit given for taxes paid to another state) on the fair market value at the time it enters Idaho.
A sale means any transfer of title to tangible personal property, either through monetary payment, barter, or exchange of goods or services. The sale occurs when a title passes from a seller to a buyer. Unless otherwise agreed, a motor vehicle title passes to the buyer when the seller delivers the goods, even if a title document is delivered at a different time or place.
It is clear that you must pay the Idaho sales tax rate on anything vehicle purchased outside of Idaho. If you pay Sales tax on the vehicle where you purchase it, you have to pay the difference if Idaho is more.
Idaho registration (plates) are so cheap that why would you want to register it anywhere else.
We live in the State of Washington and purchased a motorhome in California. We did not want to pay California our taxes, so we had a driver drive the motorhome to Nevada where we parked until we could send all paperwork to the State of Washington with the fees to register it plus the Sales Tax rate for Washington. They overnighted the plates and registration back to us in Nevada. We could have registered it in Nevada, but when we entered Washington, we would have had to register it within 30 days here and would have been required to pay the sales tax on it then.
You can not legally do what the dealer told you to do.