Please before proceeding call a (licensed trained) inspector. The house has got to be over 100 grand in todays day, whats a home inspection? 300-500 dollars? The black marks on ceiling at trusses and nails are commonly lots of candles in home. Sagging ceiling could be water from roof, if roof is dry ceiling could be re-screwed and mud and texture touch up, followed by re-paint. Good luck on house...
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