Someone has been stealing designs from Thingiverse and selling it on Ebay. Here are two videos describing the situation and how you can protect yourself.
Here is Joel aka. 3D Printing Nerd:
Here is Angus from Maker's Muse. He as a industrial designer and professional with a lot of experience suggest few steps on how to protect yourself including watermarking it and not uploading it at all:
Here is the Thingiverse post that started it all:
here is the official Thingiverse statement about your rights:
Here is the epilogue of the whole story, but it will probably happen again in a different form: