If I was in your position and the same thing happened to me I would certainly leave feedback, but as to whether it would be Neutral or Negative one has to consider the following:
- The seller is new to eBay but that should not absolve them of the need to know the location of whatever they're selling (Neutral).
- They printed off a shipping label before discovering that the item could not be located. How come they didn't know where it was? (Negative)
- There was no communication from the seller until you contacted them. (Negative)
- The explanation, when made, was pretty broad-brush: seller couldn't locate the item, wife may have sold it. The seller might just have easily discovered the item was worth far more than it sold for but "I can't find it" sounds far better than "I sold it far too cheaply". (Neutral but borderline Negative)
Feedback is just that: feedback on your buying experience. The seller may well be new to eBay but that doesn't necessarily mean they get a free ride on feedback. They may do the same in future - wouldn't you want your experience recorded in case it happened to someone else?
Neutral feedback does not count towards calculating percentage rating, Negative feedback does. Maybe give the seller the benefit of the doubt as they're new to the site and leave a Neutral? I think that would be very fair of you.