I'm sure others, way more experienced, will chime in but I would check a few things.
the belts from the motors to the gears
check to see if the gears and rails are in good shape
feel the rails and see if they are dry or have a very thin coat of grease
Do a homing sequence and listen to it while it is moving and make sure it sounds at least consistent in X and Y.
See if you can cut a larger square ( take some wood of your own ) and check the cut square's edges and make sure it came out square and the lengths are correct.
At least then you will see the entire process from Vcarve to Mach3 to Cut if you have never seen it before.
It will be a bit of work to get it apart and back together again. Probably obvious but try to label all the cables, etc before you dismantle it.
I would download and read the installation manual before hand if you can.
I couldn't be happier with our Pro and find them to be really pretty beefy machines.
Any idea why the person is selling it?