I've never been much of a gambler Nords. If someone's selling a relatively new car, then, naturally, there's a reason. I would think in the top 5 lists of general reasons, one would be that the car has been problematic.
C'mon, Az, you're scaring the rest of the class. Of course lemon problems are one reason to sell a car, but those sellers aren't that hard to avoid. Spouse and I are able to afford the premium, but that and "new car smell" have as much value to us as using the premium cash to light our barbeque. That buys a lotta longboards.
You're so focused on the pitfalls that you might be missing the opportunities. I'm not going to try to change your mind, but for everyone else here's an approach that's worked for us.
Oahu doesn't get much selection via eBay or other auction sites (yet). All of our finds have been in the classifieds, and I believe that the best (not necessarily most aware or alert) sellers still use the classifieds.
The key is to not have a deadline. If your car dies alongside the highway and you can't handle a few weeks of bus/cab rides, then you're screwed. If your repair bills are mounting or if you can hear the car's impending death rattle, though, plan to make a purchase within 4-6 weeks.
Although Snively Whiplash may be trying to unload his lemons, usually the seller's reasons defy your bargain-hunting logic. Most sellers are buying bigger for their growing family, or "upgrading" (we LOVE these people), or trying to raise money. Usually they're pushing a deadline and counting on you to help them justify their next purchase with your cash.
We usually look for cars at least three years old-- they've accumulated a significant maintenance & reliability history. We've gone as old as seven years (cherries driven by little old ladies) but we usually try to stick to younger than five. Once you pick a flexible range of models & styles for your needs (Toyota or Nissan, four-door sedan or station wagon, not "red two-year old Lexus") then keep scanning the classifieds. Find what appeals to you and set up a test drive. If the ad has been placed by a dealer, then move on to the next one. There's no need to pay their markup.
Drive the car to get an impression. This is more valuable than any other research and can save you lots of wasted time if you discover something that you just can't live with. The one item of information you must obtain on this test drive is the VIN, but feel free to check maintenance records or body condition or whatever else is important to you. If the title says "Salvage" or has a different name than the person showing the car, then you probably want to shop elsewhere. If the title still has a bank lien on it then you'll need the bank to discuss clearing the lien as part of handing a check to the seller. (We've never bought a car with a lien but it may save even more during price negotiations.) Otherwise if these problems don't pop up, then you'll be driving the car again later anyway.
After the test drive, your next tool is the library's copy of the Consumer's Report Used Car Guide. If that model of car has a crappy mechanical history then move on to another choice. If you really liked the test drive, then think hard about why you'd go against their recommendation. You may be paying for it in a few years.
If CR thinks the car seems OK, then sign up for an unlimited 30-day CarFax account. Run the VIN and print it out for the owner to review during price negotiations. Again if CarFax says "Salvage" then you probably want to shop elsewhere.
Finally, check the car's value on Kelly Blue Book or Edmunds. Again keep a printout for negotiations.
When you have two or three candidates that you can live with, you're negotiation-bulletproof. Call the owners back, drive the car a second time, and discuss with them whether you want a mechanic to look over any funny sounds or other issues. (We've never had a mechanic look over our purchase, but it might scare the seller into lowering their price. Or it'll tell you if they're trying to pass off a lemon.) If title isn't clear (death, divorce, owner's "unexplained" absence) then move on-- you have other perfectly acceptable candidates on your list.
Discuss your printouts with them (if necessary) and make an offer. Don't bargain or get spooked into raising your offer, because you have the other perfectly acceptable candidates on your list.
If the owners accept your offer, try to write a personal check or obtain a cashier's check within the hour (before anyone has second thoughts). You must conclude this process with (1) the car, (2) as many car keys/remotes as they can find, (3) the title signed by all owners, and (4) the registration. Hopefully it occurs during daylight and with license plates, but both are optional. If you're worried that these people are holding back a key then you shouldn't be buying this car from them in the first place-- you have the other perfectly acceptable candidates, blah blah.
Get the car re-titled & re-registered ASAP. One of our purchases still had a bench warrant out on the old license plate...
We've owned seven cars over the last 20+ years and only one of them was bought new. The only times we've had "issues" has been when the car reached 13 years of age... after 100K miles over the previous decade.