I wonder if its actually even worth it to do this kind of shit. Forget the ethical concerns for a minute, think about the kind of company you've built and the people you've hired to even warrant this kind of scrutiny in the first place.
It shouldn’t be difficult to track the performance of the employee with metrics that are actually specific to the task they are hired to do (Number of listings/sales).
Taking screenshots, tracking keystrokes and mouse movement? It’s extreme, and doesn’t even relate to the work they’re doing in a direct way. You might be able to see them slacking off, but it doesn’t tell you the quality of the work.