"Untested" usually means the seller doesn't have a PS/2 connection and literally can't test the board. (Relatively few peeps still use desktop PCs, and they'd need a pretty old laptop to still have PS/2.) It's a good reason for selling the board, too, right—a board they can't use?
Model F's and M's are legendary for their durability. They cost hundreds of bucks back in the '80s/'90s (in '80s/'90s dollars), and that's because they were built like nothing else. Even a board that's been used every day in a business environment for over 20 years is usually still perfectly usable—just dirty, but due to IBM's superior materials, they're easy to clean up, too.
I'm sure there's the occasional shady seller who uses "untested" as an excuse to dump a borked board. But that must be rare, for I've bought many an "untested" IBM board and found it was fine. And if you have any problems with one at all, the swell guys here will be happy to help you get it working again.