I am convinced in europe there is, with shipping outside the EU.
I am certain that it does happen, yes.
In the States, though, I would imagine it's more akin to the art theft market; steal a $1million painting to move it around in lew of moving and exchanging cash. In this case it would be relatively rare because the culture doesn't see the same value in the trees as the paintings. This is likely changing, but also not likely to affect any of us in the long term.
I could be completely wrong, though.
Elsewhere in the world, I imagine it can be much more prevalent. I suspect the Yakuza and other long lived east Asian crime rings have been engaged in illicit bonsai trade for generations, and moving rare plants in Europe has been relatively common for centuries.
Let's keep this going, because now I want to write a gritty pulp noir novel about high class bonsai thief.
Maybe in a dystopian cyberpunk setting.