If Rod Roddenberry has decent legal representation, and I have no reason to believe he wouldn't, the person currently in possession would already have been served with a cease and desist letter directing them to do nothing with the model until it is authenticated and legal ownership is determined. If they are any good at all, they would get cease and desist orders/injunction legally prohibiting them from selling the model until legal ownership is determined.
There is abundant reason to believe the model is the one Gene Roddenberry lent to the studio which subsequently went missing/was stolen. As such, the Roddenberry estate/heirs are the legal owners of the model and attempting to sell or purchase it without their authorization is trafficking in stolen goods. For an artifact of such significance and potential value in the millions of dollars, the penalties could be severe. There's no statute of limitations for legal ownership and, while often required for insurance purposes and useful in theft claims, a report to law enforcement isn't required to prove that the item was stolen.
At this point, anyone who could reasonably authenticate the model for an auction house or consignment seller is also certainly aware of its history and any reputable agency wouldn't touch it until the legal issues are sorted out. If the person currently in possession actually did something so mind-bogglingly stupid as to sell it in some kind of black market deal, they almost certainly wouldn't get to enjoy the fruits of that deal, as the subsequent lawsuit and possible criminal proceedings would probably use it all up and then some. I can't imagine someone with the scratch to buy the model would want to get tangled up in this nonsense, either.
Then again, people get really stupid when money is involved.