What the terms actually mean and how they are used are different things in many cases.
NIB - New In Box - should mean the knife has not been removed from the factory packaging. In reality, probably means it has only been taken out and admired.
LNIB - Like New In Box - should mean the knife has only been removed from the package to be examined/admired/photographed. In reality, probably means it has been handled, displayed and only used to open envelopes and the like, but not carried, scratched or sharpened.
MINT - Should mean as it left the factory. All original finish, original grinds, original edge, untouched by human hands since being packaged by the maker. From what I have seen, usually means the scratches have been buffed out. I especially loathe seeing "restored to mint condition". That is an oxymoron. It can be restored or it can be mint (original) but it can't be both.
USED - Should mean the knife has been carried and used in a manner consistent with its design, possibly having been properly sharpened as well. In reality, can mean almost anything except new. Expect some scratches on the blades and probably needs sharpening. Handles may need cleaning, bolsters will likely be fogged/scratched, pocket clips will show scratches/wear.
ABUSED - Should mean the knife has been used in a manner other than it was designed for. In most cases, it means damaged. Expect broken tips, nicked or chipped edges, loose handles, blades that appear to have been sharpened on a bench grinder, etc.
I'm sure my definitions will provoke some cries of outrage, but I call 'em as I see 'em. Some people are doubtless honest enough to decribe their wares accurately, and those people are a pleasure to deal with. Others tend to describe their wares a grade higher than they really are. In most cases, it has more to do with their pride in what they are offering than trying to bilk someone out of a few extra dollars. If at all possible, get clear, focused, well-lit photographs and examine them carefully, or be prepared to suffer the consequences of buying sight unseen from someone you've never met through an anonymous medium.