There is no appeal process for a DMV. The question is why aren't they accepting the current bill of sale? They do have to tell you why so you can "fix" it. In New York, the DMV will often require the bill of sale to be on one of their specific forms, especially if the transaction happened inside the state, meaning the seller was in New York and the buyer was in New York.
I had a two and a half year process to register my trimaran in New York. It came from the estate of my father in New Mexico. A long convoluted process where the DMV was ridiculous. Believe me, I wish there were an appeal process for the DMV...
dj