try to get it copyrighted so that it becomes ILLEGAL for them to sell it without your permission. I Don't Know the process so you will need to do the research, but this is one possible solution for you if you put in the research.
you can use a tool like http://ebooksguard.com/ it will help prevent someone from taking your ebook and ripping you off.
If you made it and it is your original work, it is automatically copyrighted, him claiming it as his own and selling it is illegal. Try to contact him and tell him you will contact the authorities if he doesn't stop. You should also always put a warning on your eBooks that it is copyrighted and they do not have permission to sell it. Remember I'm not a lawyer this is just my opinion! Definitely get some professional advice.
You can claim a quick copyright with myfreecopyright.com while you wait for copyright.gov filing (35. last I checked). Good fortune protecting your work.