Contact the business in question and explain the situation. If you have, and can prove, your continued use through commerce. You are the legal owner of that name. For example you call yourself ABC Leather and have been doing business under that name for several years. Show proof for local sales as well as Internet sales and the name belongs to you. This will hold up in a court of law. Now, you can register your business name with your State for small fee. This gives you State ownership. The same can be done with the Federal level. Now, this gives you no tax number, no right to sell. It only protects your name. Same goes the other way. When you setup an EIN or tax number to purchase wholesale it does nothing to protect your name. These are two seperate and exclusive issues. You may just find that the other person didn't know and will gladly change thier name. Or if they won't you can take them to small claims court. This only works if you are both in the same state. If it involves another State it gets dicey. I would call and go from there.