Determine the sum of all possible positive integers $n, $ the product of whose digits equals $n^2 -15n -27$.
1. We need to determine the sum of all possible positive integers \( n \) such that the product of their digits equals \( n^2 - 15n - 27 \).

2. First, consider the case where \( n \) is a one-digit number. For one-digit numbers, \( n \) ranges from 1 to 9. However, for these values, \( n^2 - 15n - 27 \) will be negative or zero, which cannot be the product of digits (since the product of digits must be a non-negative integer). Therefore, \( n \) must be a two-digit number.

3. Let \( n = 10a + b \), where \( a \) and \( b \) are the digits of \( n \). The product of the digits is \( ab \). We need to solve the equation:
   \[
   ab = (10a + b)^2 - 15(10a + b) - 27
   \]

4. Expanding and simplifying the right-hand side:
   \[
   ab = (10a + b)^2 - 15(10a + b) - 27
   \]
   \[
   ab = 100a^2 + 20ab + b^2 - 150a - 15b - 27
   \]
   \[
   ab = 100a^2 + 20ab + b^2 - 150a - 15b - 27
   \]

5. Rearrange the equation to isolate terms involving \( a \) and \( b \):
   \[
   0 = 100a^2 + 19ab + b^2 - 150a - 15b - 27
   \]

6. Notice that if \( a \geq 2 \), the left side (product of digits) is positive, but the right side becomes negative, leading to a contradiction. Therefore, \( a = 1 \).

7. Substitute \( a = 1 \) into the equation:
   \[
   b = (10 \cdot 1 + b)^2 - 15(10 \cdot 1 + b) - 27
   \]
   \[
   b = (10 + b)^2 - 150 - 15b - 27
   \]
   \[
   b = 100 + 20b + b^2 - 150 - 15b - 27
   \]
   \[
   b = b^2 + 5b - 77
   \]

8. Rearrange to form a quadratic equation:
   \[
   b^2 + 4b - 77 = 0
   \]

9. Solve the quadratic equation using the quadratic formula \( b = \frac{-B \pm \sqrt{B^2 - 4AC}}{2A} \):
   \[
   b = \frac{-4 \pm \sqrt{16 + 308}}{2}
   \]
   \[
   b = \frac{-4 \pm \sqrt{324}}{2}
   \]
   \[
   b = \frac{-4 \pm 18}{2}
   \]

10. This gives two solutions for \( b \):
    \[
    b = \frac{14}{2} = 7 \quad \text{and} \quad b = \frac{-22}{2} = -11
    \]

11. Since \( b \) must be a digit (0-9), the only valid solution is \( b = 7 \).

12. Therefore, the only number that works is \( n = 10a + b = 10 \cdot 1 + 7 = 17 \).

The final answer is \(\boxed{17}\).