I purchased a small version of this effect 10 years ago. I upgraded to a slightly larger version about 5 years ago. It was made on a small scale, and sold through a local magic shop that is now closed. I'm changing up my fish routine, and retiring the ITG. I was going to put it up on the for-sale board, but if there's someone still holding the rights, I don't want to be in violation.

Should I be concerned?

I wouldn't be concerned at all but that is just me.
I don't see the difference in selling the effect/props and selling your car as long as you are not making duplicates I really don't think you are in any violation at all.
If this is a violation then all of the yard sales I have been to are run by criminals and so are second hand stores and swap meets.
Iven

I believe this effect was invented by Gen Grant and He's long gone. I agree with irossall. Wouldn't worry about it.
--Chris