I imagine if you offer your opinion about the price in a professional context where your opinion can affect the actual value of the property then you might have to follow some rules.

If you drive by a house and say "this house doesn't look like it's worth shit" you're probably not going to wind up in court after someone complains that said house is worth quite a bit more than shit and you're driving the price down.

Laws actually can be vague while remaining Constitutional. Sometimes it's unavoidable. Court cases can be used to refine the edges.