For a non-numeric do-nothing token, I'd suggest that a minus sign (sans digits) might be more intuitive. But if you really want to use a numeric value, -32000 is a lot less likely to intersect an actual monitor.
Why do you assume that the window I'm moving will inevitable end up on both monitors of the multimonitor system? Can't I move it to the side of one of the monitors, that isn't adjacent to another one????
(And yes, I have two monitors. And yes, I'm moving the window to the left side of the left monitor. And no, it's my choice where I want my windows. And it's only mine.)