I've never found a used one to be what i'd call excellent. Sellers often don't understand why they did not like it, but there is a reason.
The biggest issue with the 24-70mm L is lens decentering, all of the internal lens elements centered properly. You can google on how to check for this, its obvious once you know what to look for.
Then I'd use FoCal Pro to run their battery of not only AFMA, but the other tests as well.
Some things you cannot fix, like the curvature of field that means the center and edges cannot be in focus at the same time. Stopping down will hide it in depth of field, but some buy a f/2.8 lens actually expecting to use it at f/2.8.