When I was recently shopping for my X5, I found that the cheapest X5s, those around the mid-$20k range were one or even TWO accident vehicles. I would get excited, see the one I want with the options I wanted and then run an Autocheck and/or a Carfax (you can never be sure, though) and the true history and reason for the price is revealed. They buy them cheap at wholesale and then price them attractively to lure the unknown public. It sounds like the one you are looking at is like this. Just be careful. As far as the warranty stuff, the above post is great advice.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim