I believe he's talking about people making "tubes" (backgroundless images)  out of ready made props.  Basically opening Poser or Daz}|Studio, loading up a prop of a tree or a rock or a chair etc, and then redering it and saving it as a .png with a transparent alpha channel so that the image can be post worked into any image.

He's saying that allowing people to upload such items here is taking away sales from merchants selling those particular items. Basically why would someone buy the mesh when they can get their hands on a rendered image and then just work it into their image in post work.  Essentially why spend $5.00 on a lounge chair when you can get a picture of the same one that you can use in your post work, for free.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi