The BMW is just an example, but is this really a good buy? Well, the one in question (link will expire in a few days so I won't bother posting it) is a 530i, which in 2004 was the best 6-cylinder option one had in a BMW 5-series. So there's one "pro" to that purchase. It also has 30K miles - another excellent "pro", indicating it hasn't been used much in over five years of ownership. It's in immaculate shape - another pro.
But it's a 2004! 2004 was the first year of the newly designed 5-series. Is this a bad thing? Not always, but a few things to note about new body styles:
- In German vehicles in particular, they may carry the same exact engine as the prior model, at least for a year or two, tweaked for more torque. This happened to the E320 (1997-2006 had the same engine). This isn't necessarily a bad thing; in fact it can give you confidence that the engine is tried and true, and may be a reason to get into that newer body style if you like the design better.
- New body styles sometimes come with new electronic wizardry - ventilated seats, new traction system, new wiring harnesses, iPod integration. This has no doubt been tested extensively at the factory, but over thousands of units being shipped per month when a new model comes out, there are bound to be issues. On the used market, this is particularly a problem as the vehicle likely isn't under warranty and some of the nuisances may not be covered under any affordable aftermarket warranty.
- Sometimes, companies make missteps. The current-gen E-class is a nice looking car, but I would honestly rather own a "W210" chassis (prior gen). It seems more spacious, is more classic in feel and design, and the layout makes more sense than the W211. That's not to say one shouldn't buy an E-class from 2003-2009 used, but shop it against comparable models first, or look for concept photos of the next generation to see how radically the company is changing the style. Do research and comparisons to find out if that's the car you really want for a bunch of years.
- If the old body style had features that were nearly as good (a la the old vs. new E-class) and the design differences don't bother you so much, maybe look at the old body style if it was considered more reliable, depending on age & mileage of course.