How would you prove the trace of a transformation from V to V (where V is finite dimensional) is independent of the basis chosen?
The simplest way it to note that a basis transformation of a transformation $T$ is done via $ATA^{-1}$ where $A$ is an invertible matrix, and that the trace has the property $\operatorname{tr}(AB)=\operatorname{tr}(BA)$. Putting this together, you get $$\operatorname{tr}(ATA^{-1}) = \operatorname{tr}(A^{-1}AT) = \operatorname{tr}T$$