Unused condition, the original box and paperwork, a crisp readable tang stamp, an unsharpened factory edge, and scarcity of the specific variation. What destroys value: sharpening, carrying, cleaning aggressively, and losing the box.
Mint, unused condition above everything. Then original packaging and paperwork, which for many collectors is close to half the value on a modern piece. Then a crisp tang stamp that establishes identity and date. Then genuine scarcity of the specific variation, not just the model.
Handle material matters too: on traditional knives, well-matched stag or premium jigged bone commands a premium over standard covers.
Sharpening, permanently. Carrying, which produces pocket wear on the bolsters. Aggressive cleaning or polishing, which can soften a stamp and remove patina collectors value. And losing the box.
Well-meant restoration is the classic mistake. A knife polished bright by a proud owner is usually worth less than it was dusty.
Decide at purchase whether a knife is a user or a collectible and never let it be both. See user versus collector.
Keep the box, keep any paperwork, and store it properly. See storing a collection.