i too made the same mistake when i signed up with them. i used my real name as my DBA, and a couple of my old buyers saw that and called me out. i quickly called up merchantinc, and requested a DBA change. they send you a form and ask for some sort of proof of the new DBA. i used my ebay user name, and i just faxed them a copy of an ebay page with user name on it. the whole process took a few days.
i did get a tax id number under a new DBA, but i have not used it yet. the IRS allows one tax id for sole proprietorship if you are incorporated. after that i think you would need to legally start a new business to get any new tax ids. the first one all you need is a ss#. the tax id is free too at IRS.com.