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With the recent run-up in gasoline prices and the popularity of biodiesel, there's a '''lot''' of shady characters selling diesels out there! Beware! Older diesels are often listed for '''ten times''' or more the price of similar vehicles with gasoline engines! Are they worth it? '''RARELY!'''

# Best thing to do is to pay a reputable diesel mechanic to check over a vehicle for you. If you're considering paying over $1,000 for a vehicle that is over 20 years old, doesn't that make sense? 
# Carefully consider the running gear. Sure, a diesel engine may last a long time, but if the rest of the vehicle is slowly falling apart, it may not be such a bargain. Carefully inspect transmissions, exhaust systems, electrical, steering, brakes, suspension. Any one of those needing work may still be a bargain, but if multiple ones are hurting, make sure the price is right. Demand to see receipts for claimed recent work. 
# Beware the "recently rebuilt" engine! Don't let this jack up the price unless you see a receipt from a commercial shop verifying the work done, the miles at the time, when, and how much. "Rebuilt" means many things to many people. Was the bottom-end replaced because of a knock? Was the head replaced or re-tooled? Or was the engine torn-down and rebuilt, with every major surface inspected for wear and replaced as needed, like they do with commercial diesels at 600,000 km (375,000 miles) or so. Also important; '''why''' was it rebuilt? Was it just tired after many miles (in which case, double your attention to #2, above), or did it throw a rod? In many cases, the '''need''' for a rebuild indicates the vehicle has been abused, and there may be other, still-lurking problems. 
# Bring a big piece of cardboard and a head lamp. Crawl under the vehicle and look at all the seals -- the parts of the engine, transmission, and differential where moving components come out of. The engine oil pan should be relatively clean and dry. If it is caked with road grime and drippy, you've got a leaker. The seller may say something like, "It only uses a little oil," but a leaker is a '''symptom''' of other problems, not the real problem. '''Especially''' don't buy a leaker that the seller claims was "recently rebuilt!" Most scam artists will be astute enough to clean up the topside, but will be too lazy to clean up the bottom! 
# Listen to the running engine. It should have typical diesel rattle, but it should be smooth and not have other periodic sounds. Run it at various RPMs and listen for changes. If it has a turbo, be sure you take it on the highway and listen carefully for strange sounds at hard acceleration. 
# Use [http://www.kbb.com Kelly Blue Book] pricing. Print out the page for the vehicle and bring it with you. It is not unusual for older diesels to be listed at four or more times the KBB "Excellent" private-party price, but it damn well better be a perfect vehicle in such a case. If the seller poo-poos KBB pricing, walk away -- they should at least be interested in knowing if their vehicle is priced way out of line with regional averages. 
# Don't fall in love. So what if it's the only '80s small 4x4 diesel pick-up you've ever seen! Does that make it worth $3,000 when everything is leaking from the "recently rebuilt" (by the seller's brother-in-law, no receipts) engine, the "recently painted" body looks like spray paint from Fred Meyer to hide rust, and KBB says it's worth $1,200? Learn to walk away. 

That's all for now. I've recently looked at a half-dozen diesels from Craig's List, all were described as "clean" or "rebuilt," and all but one had '''major''' problems that the seller was either ignorant of, or purposely dishonest about. (I bought the one out of six that was actually as described!) 

Good luck, and remember: the only way the scammers win is if you buy from them!