I thought it was customary for woodwind players and horn players to play instruments in music stores before buying the $3,000 saxophone, clarinet, or trumpet? I think the mouthpieces are simply sanitized after testing the instrument. Every saxophone, even those of the same model, can sound and feel different to a discriminating player.
I can understand most store owners don’t want to go to this trouble to sell a $50 melodica.
When I first started purchasing used melodicas, I took them apart thinking I would find them filled with dirt, mold, dried saliva, and other spooky stuff. Every used melodica I purchased was basically pristine inside. Some surface corrosion or patina on the brass reed plates, but nothing that scared me, and I am a germaphobe. That was a complete surprise to me.
I have, however, purchased a couple of used melodicas that smelled like an attic inside. I simply disassembled these melodicas and washed all surfaces with soap and water. (I thoroughly clean every used meodica I buy just because that is the way I am.) The attic smell would not come out of one of these melodicas even with washing, so I soaked the whole thing, fully assembled, in a mixture of water and vinegar for a few minutes. I then submersed it in a water bath with dish detergent and then rinsed it thoroughly with fresh water. I drained the water out of it and let it dry. It smells and works fine now. Do this at your own risk. It worked for me on one particular model, but may not be suitable for other melodicas. Since melodicas get wet on the inside from condensation, I assume most of the materials (gaskets and adhesives) are water resistant. That is my assumption anyway. I have no facts to back it up.
My local accordion technician, who has been repairing accordions for about 45 years, said that brushing the reeds and reed plates with rubbing alcohol can remove dirt and buildup and can improve tuning and performance.