Is there any potential problems buying used diff gears, and can they be re-coed or whatever to get them back to a near new condition. Looking at a set of gears but seller says it may produce some gear whine.

You can get them " Lapped ".....there is a mob in Brissy that does it.
Or, you can fit them up with all new bearings & seals, and run some Penrite Limslip 140 in it.
This is what I did....had these diff gears in 3 different housings now, and been bolted to 2 different centres as well....and the whine is very slight ( need radio off and windows up to hear it ), on decel, and coast.
Its the luck of the draw I suppose......just as long as they are not rusted/pitted.