Good list of criteria. If it’s a recent model, chances are pretty good that if it looks good in the photos it will be in reasonably good shape and playable. Photos can be misleading, however. The vendor I think MM is referring to takes very flattering photos of his stock. If it’s a vintage model (1980s or earlier), there’s always some risk involved, and my experience with older models is about a 50% success rate. Some vintage models are much better than others, and it might be good to work up a list of the best vintage melodicas to buy in terms of playability and durability. It’s generally a good idea to ask questions of the seller: Do all the notes play? if you blow into it with no keys or release valve depressed, is it relatively air tight? Whether it is odoriferous is not so much an issue, since it’s not hard to deskunk a melodica. In photos, look very carefully for cracks and discoloration.
There are certain items that I’m willing to buy because of their rarity and historical significance, whether they’re playable or not. That’s because I’m interested in the history of the instrument. But rare and exotic melodicas are often not very playable. And if you’re looking for an alternative to a popular melodica of the past, such as an Italian or Czechoslovakian counterpart to, say, the original metal Hohner Piano 26, hoping that it might be better, the fact is that these models typically are not as good as the models of which they are imitations, and are less likely to be in good playable condition.
Certain highly desirable vintage models such as the Claviettas and the Hohner Professional 36 typically need work.
I have not been able to find a melodica museum or harmonica museum that includes a serious melodica wing. Those of us who collect probably should be thinking about identifying a future museum repository for some of the significant items in our inventories. I have some ideas about this for possible further discussion.