Formula for percentage increase or decrease when one factor is zero

You can't divide by 0 in mathematics - it's illegal... You need to trap the condition using an If function.
e.g

=IF(Previous Inventory>0,Do a normal calculation, Do something else)

Criteria to trap the error
Normal calculation
Alternative Calculation

In reality it looks something like : =IF(J7>0,(J8-J7)/J7,"0+" & J8)
where
J7 = Previous inventory
J8 = Current inventory

When it sees a zero inventory it displays the absolute increase in stock rather than the percentage