Just one problem with buying used, the rubbers or plastic bits can get brittle with age. I'd just think about adding new foam pads especially if you get one that hangs on your tailgate. Also the claw clamps will get brittle and split over time.
Having said that, I split one of the claws off my Thule rack after only 3 months from new. But got a new one obviously under warranty.
But they are stupid expensive. I would buy a more basic halfords one if I had to buy again (I've a tow bar mounted one).
If going used, just give it a good look over.