I don't have experience with a purchase/sale agreement, but don't hand over money until you have run a title search. AOPA offers that service to members for $99:
http://www.aopa.org/info/certified/tne/TitleSearch.htm

Also go here to request a CD from the FAA that contains all records (registrations, mods, etc) that they have on file for that aircraft. The cost is $5 and they'll send you a bill. It's the best $5 you ever pay the government:
http://162.58.35.241/e.gov/ND/airrecordsND.asp

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