Purchase and Sale Agreement

Or you come up with a price up front and stipulate in the agreement
that any airworthness issues are either payed for by the seller or
reduce the purchase price if they exceed some negotiated dollar value
that differentiates things like a bad plug from a cracked engine case.

As I mentioned, that is the problem. There are things that I may not
like about the airplane (maybe I didn't realize the Garmin 430 was VFR
only, etc) that after the pre-buy I may want to adjust the price on
that have nothing to do with airworthiness. That would support the
arguement that you should wait until after the pre-buy to negotiate
price.

-robert