Point of clarification: Most of the comments posted so far apply to buying an older item, usually referred to as “vintage.” There is not much risk in buying a current model that has been used so long as it looks okay in the photos. I’d say that this isn’t a bad way to buy if total cost including shipping is about 60% or less of the cost of buying new.
The question of value is more complicated with vintage melodicas, since it varies considerably with models and it depends so much on the condition they’re in. Also, rareness (determined by how many were sold) is a factor that is commonly not reflected in the seller’s price. For example, it’s easy to find a 1970s Suzuki Study 25 (A-25), but its contemporary 1970s Suzuki A-26 is much rarer — both very good Melodions). Consequently, the A-26 should (in my opinion) sell for at least twice as much. But this is not reflected in sellers’ prices. (These comments do not apply to certain exceptional vintage melodicas such as the Clavietta or Hohner Professional.)
Playability of vintage melodicas is an issue for further discussion. Not all vintage models that are in comparable condition are equally playable (or “gig worthy,” as Melodica-Me has put it), They probably were not equally playable when they were new. And some have sound or other features that distinguish them from current (2015) models.
I can say from experience and from my own mistakes that it’s wise to read up (here) about vintage melodicas and to follow melodica auctions for a while on eBay (at least) before jumping in.