The LM324n has a gain-bandwidth product of 1 MHz, so I guess it works from 0 Hz to 1 MHz, is what I understand true?
Its slew rate is 0.5V/us which means that it can generate a 500 kHz output signal, is that true too?
If the above are both true, why when I apply a 75 kHz 12V peak signal to Vin+ with +12V DC supplies and Vin- is connected to Vout so it should work as a buffer, is the output signal destroyed?
There are a couple of problems with what you are trying to do:
The slew rate is not sufficient to pass a 75kHz 12Vp signal. The max rate of change of your input signal is the max of the derivative of 12*sin(wt) or max(12*w*cos(wt)), which is 12*w or 5.6V/us. Look at Fig 11 in the datasheet.
The max output signal swing is (typically) the supply rail minus 1.5V, so a buffer with +/-12V supplies will not be able to reproduce a 12Vp sinewave.