That maybe there was a problem before or there was work done while it was sitting. There is not a lot of things that can cause a starter to spin and not engage. Are you sure it's the stater you hear spinning? no grinding, clicking or clunking? Is the engine definitely not turning? Can you check the voltage at the starter?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)