My feeling is that it is the seller 's responsibility to provide the Certificate of Occupancy for the basement. Many times, a basement is finished to cover water issues, mold issues, foundation issues, electrical, and the list goes on.
Most likely, the Town will be happy to work with the seller to achieve the C.O. in order to close.
There may be a few bumps in the road that both the seller and the buyer may have to address, but if the the house is ideal for you and you really like what the basement is offering....
I would dedicate the time to see what the town's perspective is....
Perhaps, the town will pass the basement with some simple documentation from the original contractor.
The fees involved are rather nominal in the whole scheme of things.
The main objective in a situation like this is obtain a C.O. to ensure that there are no defects , no hazards that may be life threatening and equally important is to have the ability to sell the home in the future with a 100% marketability status.
Hope that this helps!