Regarding the steps that you need to take to inform Montana that you sold your vehicle, there are essentially none.
You don't need to submit any forms or officially notify Montana of the sale of your vehicle. Montana simply requires sellers to remove their license plates from the vehicle when they sell it. Why? So that parking tickets, tolls, etc. can't come to the old owner.
Montana simply requires sellers to remove their license plates from the vehicle when they sell it.
If you're traveling in Mexico, Central America, or South America, you don't need to worry about the buyer racking up tickets or tolls on your behalf - any tickets or tolls will be due immediately, not sent to the vehicle's registered address to be paid later.
And so, because the vehicle on the PanAm is almost always on a TIP that is tied to the license plates that the vehicle entered the country on - and because switching license plates will cause them to be mis-aligned with the TIP - allowing buyers on the PanAm to keep your plates (at least until they are able to enter a new country and switch onto their own plates) is advisable.
If you're traveling on the PanAm, you can let the buyer keep your plates.
If the plates hold sentimental value, you can always arrange for the buyer to send them to you after switching onto their own plates.