They're returns rather than broken and fixed machines I understand. And they are personally tested and inspected by a human, unlike the ones that roll off the production line.
Only risk I can see is if your machine has any scuffs from the previous owner.
IMHO something which has been used for months by various people and probably had a problem, it's going to have problems as soon as it can.
And risking this just to save few hundreds dollars (on a Computer that costs 2000 or something) isn't worth the risk.