I'm still unsure why you are going through Maryland for this. You're in OH correct? So to register it in OH print out the OH Bill of Sale, fill it out completely (minus the signature), put a post-it on the line where the seller needs to endorse it, include a letter explaining the issue, include return envelope & postage, and send it certified to the seller as you have all their info.
They should get the point then. Otherwise you'll need to chat with the DMV to see what they actually require after explaining the entire situation again.
There may be other avenues, ie. court, depending on what your state allows/requires.