What makes something desirable is very subjective, and I don't think we can be dismissive of one person's idea of desirability simply because it doesn't fit in with our own perceptions. Desirability can come from the accepted monetary value of an object, from a sheer admiration for the object, from what memories the object evokes, or from the challenge it provides in completing a collection. It only takes two people to want a rare object for any of those reasons to make it financially valuable.
Concerning this particular Diana pistol, it probably only fits into the category of 'completing a collection', lacking any other virtues. But to the collector with a particular interest in the history of the air pistol, it will be desirable, and he will not have to be 'persuaded to buy it', though he may need to be informed that it exists.