Would be about right if the frame was in good shape. Most people are only giving $5-6K for 450s outright, and I understand he is asking $15 because of the K but realistically it’s not worth the $15K
Not sure if you’re still looking, or if this is still available, or if a sleeper cab would suffice, but I just found this: https://www.truckpaper.com/listings/trucks/for-sale/199941411/1973-kenworth-w900a?CTRY=USA