I have used Craigslist without issues. Easy and cheap. Autotrader is overrun with dealers around here. Kijiji is popular but I haven't used it.

Don't understand why you would ask here. This isn't an X5 question. What works in your neighbourhood? Ask locally.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue