Question

Why in equilibrium the bulk modulus is always positive

Answer 1

From the actual definition of absolute value, 
the absolute value of a number is its distance from zero. 
Now,  can the distance be measured in negative values?  Is there such thing like,  x is - 7 units away from zero?  
No,  right!  Which is why absolute value is always positive.  
On the real number line, for the absolute value |-3|,  measure the distance from zero to - 3. You will see its 3 units.