the one you are watching on ebay though, make sure the spring is tight on the flip section

lots of them sag over time due to the springs getting knocked about and even though they are designed to bounce back, once the spring stretches beyond it's normal rate it loses it's elasticity and thus you get the sag

if your mirror isn't sagging though and you just need the innards, they are fairly easily swapable as the motors are screwed into place on the little housing, one motor moved the mirror up/down and the other moves it side to side (there are only 2 motors and they simply wind in/out and when things break it's normally the little plastic lugs that have the teeth on them that get striped)

post up a pic of the damaged one and i'll walk you through if you like