PS.... I wrote:
So for short sales, the annualized IRR is (1 + (s-p)/s)^(365/days) - 1,
where "s" is the initial net sales proceeds, "p" is the later net purchase
proceeds, and "days" is the hold time.

For your example, that would be about 615.56% annualized IRR.

In case it is not obvious from the description, "s" and "p" are both
positive typically. That is, they are __not__ signed cash flows.