How much are you paying for used fans? Hopefully not more than $5. At that kind of price point, it is basically disposable and completely RNG based as to what you'd be getting. 99% of the time the stuff should work, but the bigger question is will they work well enough for you? Will they be quiet enough, will they move enough air? Is the fan bearing going to last long enough for you? If you don't mind then it don't matter. Then again if you do care, why are you cheaping out and playing the lottery with old fans? Too much work to save so little money.