If the instrument is one I intend to play versus collect, I ask the seller if all the notes work and does it leak when she blows into it fairly hard without any notes depressed. If it is a third part seller, I ask if the melodica smells like an attic.
If the brasswork on the case is corroded, I know the melodica has not been stored correctly and will not buy it. (Chances are the brass reed plates are distorted too.) If the case looks like it has been cleaned vigorously and there are still signs of stains, mold, or excessive dirt, I will not buy it. If the screws are marred, I will not buy it.
All of the above goes out the window if in fact the melodica is one I consider to be rare.
If the photo of the instrument was taken on an embroidered kitchen table, being sold by a woman, and meets the above criteria, I buy it. When I get it, it will smell like perfume and I know it was played for a few months and then stored in a camisole drawer for 50 years.