One thing to lookout for is a "ghost" preparer. This means they could be unqualified and promise a big refund then charge fees based on a percent of it. They may even doctor your return to fraudulently allow for a larger refund.
The problem is, you have no one to go back to if there is a problem.
Many times "ghost" preparers get away with this by not signing the return or including their tax preparer ID number, therefore the IRS doesn't know they exist.
To make sure you don't become a victim: