Using a computer thatís not yours, you can set up Craigs list account and advertise cars for him. Make sure you list prices below his. Offer luxury cars with shit loads of amenities at a price that will have lots of people coming in. Also as announce discounts for various people (honor roll students, retired government employees etc.) When people come in and realize theyíre not getting any type of deals, his sales will drops. They too will probably do things to get back at him.
The ole rumor mill can be your best friend. Let it known that heís using substandard car parts. If his car lot has a garage, a rumor of a chop shop will get the cops snooping around. That junker in the middle only has 17,000 miles on it? Itís only possible if he turns the mileage back, which is a big no-no. If heís selling newer cars I donít think you can change the mileage. Iím sure there will be investigations. Even if the authorities have no evidence, they will keep an eye on the mark a little while. So will people who are looking to buy cars.
The next revenge trick will be expensive on your part but I think rip-offís customers will be happy when they buy there auto and the gas tank is full. Keep in mind that the sugar in the gas tank trick has been proven to be ineffective. The MythBusters did this on their show. Keep the sugar for your morning coffee. Soooo you get some diesel fuel and pour it in the tanks. I never tried this trick before so Iím not sure of the results.
Another trick is to get some roofing nails and prop them against the tire. Do this to all 4 tires.
The oldies but goodies include bologna on the car, when itís taken off the car will be polka-dotted. Brake fluid ruins the paint, while urine poured down the slots near the windshield will result in a nice fragrant vehicle.
If you can have someone distract Mr. rip-off, you can get access to his phone and have any incoming call forwarded to a competing dealer. Not only will he have no calls that day, youíll save some people from getting ripped off. You can also setup a beige box or some device to intercept his calls. You can prank his customers to the point of them going elsewhere for their auto purchases.