Physical things to look for:
Panel integrity. No dimples, bends or distortions on the panels.
Diaphragm tension, is the Mylar nice and even (no wrinkles). Wrinkles are a sign of loss of edge integrity on the panel. New ones might be needed.
Box integrity: any cracks in the woofer box, usually in the rear, as dropping the crate or box they are in can put big stresses on that part.
Audible things to listen for:
Using a sound pressure meter, both speakers should be within in a DB of each other.
Even better, disconnect the binding straps between the rear input posts and listen to just panels, then just woofers and see if you hear any distortions or colorations you’d object to.
Comments
Almost any SL3 is now old enough to probably be candidate for new panels soon. So factor a $550 panel job into your future expenses. You might be able to delay it for a few years, but it’s coming.
But that’s the great thing about ML’s, for reasonable money they can be returned to as-new (or even better than new) condition pretty easily.
And compared to new costs, they are a bargain.
Good luck with the purchase decision.