Roush used or new

I have heard it depends on the manufacturer of the gears, I bought a set from a generic company and they whined for the first 10K miles due to the inexact cut of the gear. I Later learned that the Ford Motorsport gears do not whine. As far as the gears go look at each tooth very carefully for chips or nicks, If there all clean your probably fine. Specially if there free! :)
 
In my last two cars I have put used gears in them...or had them installed, I didn't do it..and they worked fine in both cases. Just check for chipped teeth and make sure the wear pattern is good and you will be fine. If they are worn uneven or on one edge, or you see chipping on the teeth just toss them. Otherwise go for it.