Got a freshly slabbed slab off feebay and low and behold there's a scuff on the case above the label
Was able to remove it using plastic polish from an auto supply store (normally used to polish foggy headlamp covers).
First, I wiped the area with a microfiber cloth to clean and make sure it's an abrasion, not a smudge.
Used a paper towel and a pea-sized drop of polish, no pressure, with circular and side-to-side motion then wiped the residue off with a clean paper towel. This plastic is soft so it only took 3 rubbings to get the job done.
Apologies if this is common knowledge. These cases scratch easy so no need to reslab if you have a heavily abraded case. You could even take a buffing wheel to it if you're hardcore.
NEXT UP: waxing a slab! Furniture wax? Car wax? Not sure when I'll do this so if someone else wants to take the plunge...