Try Carmax. We've sold our last two vehicles to them for a reasonable price and less hassle than doing so ourselves. Also, you can try posting on this site (if you haven't already) or the BMW CCA classifieds. Note that if you aren't a BMW CCA member, you'll have to pay for the ad.

Geez I hope the X3's value won't fall like this, although I don't see it happening in the foreseeable future.