You may be able to get a roofer to quote a replacement estimate high, and then replace the roof with a more reasonable company and have insurance money for your pocket.
Or, make a counter offer to the buyer with 1/2 of the total cost of the roof removed from the offer. You may be surprised.
I beat up the guy I bought from too. A 15 yr old roof may need repair, even if no wind or hail damage, and should be factored into the asking price. If you ask high $$$ per square foot, your castle better be in top shape.
FWIW, I re-roofed my old house before I sold it. That way, there was no way it would be an issue. Hell, the only thing they found was double keyed exit lock in the garage to beef me on.
If you are capable of reroofing yourself, kill the escrow, reroof, and relist. People never seem to have the time to do things right, but have enough time to do them again.......
I also reroofed my new house. 15000 nails, if you know what I mean. Cheap to do, just ass breaking work.