If you create a tree model "de novo" using the vertex and/or spline modeller, you can distribute or sell it. But my guess is that you can't sell or distribute the tree primitives (assuming it's possible to export them), because it would mean that people wouldn't have to buy Carrara to use Carrara tree primitives. Meaning that somebody could just set up a "Carrara tree factory", churn them out and sell hundreds of them on Turbosquid for $300 each..... hmmm, maybe I better call my accountant.