Gears & Axles - Used Gears?

I see alot of people selling used gears on here. I was just curious to know if this is ok to reuse gears? For instance they say stuff like 3.42 gears 20k miles no whine, etc, etc. Can you really reuse gears with no problems at all? I would think it would be hard to get the wear pattern to match up again? Thanks for the help,

Is it ok? I think you are right -it will be very hard to match the wear pattern.The gears may have been quiet before but will probably whine in your car.So its a matter of some noise versus $ saved with used gears-your decision.

awesome thanks for the reply