If the first 3 pages of the top selling shirts on Roblox can be considered a random sample of all shirts on Roblox (which they cannot), approximately 45% of the shirts are stolen.
This is still an estimate. It's probably slightly more accurate than 90%, but still meaningless.
In order to get an accurate figure, you'll need to write a program that randomly chooses about 1000 shirts, and then have a trained/educated user determine whether each one falls into the category of stolen or not stolen. At best, they can still also only make an educated guess - they don't have the time to search for a 1000 images on the entire Internet, let alone a single one. This would introduce a huge bias. As an example, when I evaluated which shirts were stolen, and which were not, I used the following exteremely qualitative method:
Does the picture look like a photograph?
Does the picture look like clipart?
Does the picture look like a screenshot or digital image?
If the answer to any of the questions was "maybe," I counted it as stolen, with a few exceptions. If it was a screenshot of Roblox, I gave them the benefit of the doubt and assumed they made it themselves.
So, without a computer with a huge library of images stored on it and a program that compares images, there isn't a good way to detect every stolen image.
However, while looking at shirts, I noticed a bunch of extremely recognizable brand names that the average consumer would be familiar with. It's a very safe bet that they are used without permission. If you can look through 1000 images, and pick out all the ones that say Pepsi, Nintendo and Enron logos plastered all over them, and come up with a figure, say 2%, you could probably soundly make the argument that at least 2% of the shirts were stolen.
In any case, you shouldn't be able to sue Roblox, you would need to take the individuals to court. This isn't economical, which is why in most cases businesses like to fall back on the dreaded cease and desist order (see ytmnd, something awful, et al. for reference).
A better solution would be to encourage users not to infringe on other's copyrights, and there are a few very simple ways it could be done:
o Further stressing in the Terms of Service (more so than it already is) that users do not upload content they do not have permission to use.
o Set up a system for copyright holders to request their content be removed.
o Provide an incentive for users not to steal content. Perhaps offer tickets or other prizes to users who report copyright infringement.
