I found a site selling t-shirts with two of my designs on. They’d simply taken the diagrams from my site and started selling them! I had a protracted e-mail exchange with the culprit, who initially claimed they were his own designs!
Eventually he ‘fessed up and to be fair, removed them from the site. You just never know what’s going on behind your back, that’s why the Origami Artists & Creators are so important, helping to police things.
There seem to be an endless number of sites who allow you to upload images and generate income from various types of merchandise. You may have seen Dave Venables impressive merch site.