Here is my take.
It's used. It may be in perfect condition. It may have had it's fluid changer every 10,000 miles.
It may have beat to heck. It may have never had a fluid change. It may be full of water and rusted to death. The actuator may not work, there is a few hundred dollars.
IF I ever bought a used diff, I would have it rebuilt and checked out before install.
I have a friend that bought a "good condition" used complete unit and in a short time he had issues.
For $750 you can buy a new ARB. True you will have to get a compressor, install kit and have it installed but you will have a locker that will instantaneously lock and unlock. and you know that it is new good parts, including seals and bearings. An e-locker will never actuate as quickly and seamless as an ARB.