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There is not much to add to the other answers. Your code is well written and easy to understand, but as a brute force algorithm it has its limitation in real world use - for larger data sets at least.

The name should be MaxPairProduct (PascalCase) instead of maxPairProduct.

There is maybe an issue (that applies to the other answers as well):

For the dataset:

int[] data = { 1, 2, 3 }

it returns 3, but it should return -1, if the multiplier and multiplicand must be different from the product itself? The dataset { 1, 2, 3, 3 } should return 3.

Similar

int[] data = { 1, 2, 2, 3, 4, 5 };

should return 4 but returns 5.

The above is somehow inconsistent with:

int[] data = { 2, 3, 4 };

that correctly results in -1.

Below is an implementation that handles the above problem, and also prevent a possible overflow when multiplying:

static int MaxPairProduct(int[] a)
{
  Array.Sort(a);

  int product;
  int multiplier;
  int multiplicand;

  for (int i = a.Length - 1; i > 1; --i)
  {
    product = a[i];
    for (int j = 0; j < i; ++j)
    {
      multiplier = a[j];
      if (multiplier == 0) continue;
      multiplicand = product / multiplier;
      if (product % multiplier == 0 && Array.IndexOf(a, multiplicand, j + 1, i - j - 1) > j)
      {
        return product;
      }
      if (multiplicand < multiplier)
        break;
    }
  }

  return -1;

}