'How to buy a used car' by Bikewrecker!

Tampering with the engine:

Get out your trusty screwdriver while the guy's not looking and remove the distributor cap. Stuff an old rag in it, and put it back on. If you don't want to risk a rag, you can use a graphite pencil and draw conductive lines inside of it.
Pour some ATF down the carburetor so the engine will smoke when he starts it.
Re-arrange the plug wires.
Pull the fuel line off, that way it will run for a minute and then die.
Cut the wire to the starter solenoid, so it won't start.
Squirt some oil near the valve cover gasket and then 'find the leak'.

Unplug/cut any neutral safety wire. This will prevent the car from starting.
While you're under the car, jam a scrap of wood near the shift stick so it won't shift properly. This is best done with a manual transmission with a side shift linkage.
Unplug the speedo cable. Don't forget to show him how the speedo isn't working during the test drive, and how this means the car must have more miles on it than the odometer reads.
Pour some oil under the transmission, so there'll be a big slick there when you take the car for a test drive.

Take along some large hoseclamps and some small pieces of scrap metal. Clamp the scrap to the driveshaft. Be sure to point out the vibrations while you're test-driving.
Put on a whole bunch of stick-on mag wheel weights on the inside of the wheels. Again, point out the vibration.