I did mention that if there is an "issue" that you know about, then your are honor bound to disclose it.
However, there can be issues with a knife that you are unaware of, and the knife would still be NIB. how so? I'll explain.
I was into sports card collecting when I was younger. Perfect example. With sports cards, if a package of cards is unopened from the factory, every card in that package is considered to be NIB / mint condition, even though it may not be.
In 1989 Upper deck released it's first box of baseball cards, and the number one card in that box was Ken Griffey jr's rookie card. Now that card being #1 in that box, meant that it sat at the end of the box, against a cardboard spacer. That cardboard spacer in some cases would slightly discolor that #1 card. BUT if the have one of those sets (I do) and the box has never been opened (mine hasn't), then that card and all the others inside the box are considered Mint condition. If you opened your box....... now all of the cards are subject to being individually graded for condition.
"Ken...... how does that pertain to knives?"
Well, it is not common in knives, but some manufactures will but their knives in sealed boxes. Such as one of the current HOT knives on the market right now,The ZT 0454. That knife is coming from the factory in a factory sealed box, the seal even says.......
"STOP, if seal is broken, check contents before signing."
So if your purchase one of those knives and leave it factory sealed, IMO that is a a NIB perfect condition knife. If you decide to open and look at it. (how can you resist?Now the knife is subject to inspection and reveling it's faults, to a potential buyer.
I understand the broken seal theory pertaining to something that is potentially collectible, but for the price people are asking for them, I would want it opened so I knew EXACTLY what is inside....
Manufacturers have humans doing some of the work, so mistakes can be made. How many times have we seen a knife for sale that doesn't match the box labeling ? It would be very disappointing to pay big money for something that was sealed, and a few years down the road decide to open it and admire it, only to find out it wasn't what it should be

I have always tried to be cautious about how I describe a knife or other item I am selling, and if anything, under rate it, just so there are no surprises. That said, I will be even more cautious in the future when I list something for sale. I take the best pictures I can which certainly aren't up to the level of many here, but they do clearly show everything.
Even at that, we still have to remember, " The deal isn't finished until both parties are happy ". However, buyers remorse shouldn't be something that a seller has to accommodate.