The Title Search is one of the most fundamental items in the purchase of an aircraft. Ensure that the aircraft is not encumbered by unpaid taxes, mechanics liens, and unpaid bank loans. There are other less common issues but the key here is to know before you buy.
If you buy a plane without verifying a clear title, you the new owner just assumed all the financial liabilities associated with that aircraft. We have seen encumbrances on a title far exceed the value of the aircraft.
Common things which may encumber the title of an aircraft are:
Mechanics or Artisans Liens
Unpaid Fuel or FBO bills
Pilots and crewmember claims for pay
Court judgments against the current owners
As a side note, Plane Fax can also lien aircraft for you at the FAA if you hold a valid claim.