When a prospective buyer contacts you and wants to test flight the airplane if airworthy, ask to see his/her Airman's Certificate and establish the true identity of the person-Driver's License, etc. Run copies of those. Establish whether he/she has retractable gear, complex aircraft experience. Have a short meeting with the prospective buyer and the airport manager. Let the manager know there will be a test flight (that you must go along on), mainly in the pattern or close area-when and about for how long. The pilot should call for a local weather report as a minimum. A complete pre-flight per the specific aircraft model checklist should be done by the prospective buyer. You witness and verify that all steps comply, including sumping the fuel tanks for water, fuel & oil levels, etc.
These cautions considers the possibility of hijack theft of an airplane-the last thing you need. A prospective legit buyer might also bring his own Airframe & Powerplant Mechanic to verify aircraft condition, Aircraft Directives compliances, Service Bulletins-(not mandatory) etc at his/her own expense. You establish the mechanic's credentials also. The mechanic will want to ride along also as the Super Viking will hold you and his mechanic.
If you need a mechanic to get the plane airworthy, a one time ferry permit from the FAA might be required. I know a Bellanca Viking specialist A&P in SoCal who gets owner pilots from as far as Texas or more.