I have purchased several accordions on Ebay and the minute I remove it from the box, the case tells the story!  Ask the seller if the case shows any evidence of moisture, water stains.  If the case got wet or endured extreme moisture,it may be an indication that the accordion will not only smell awful, but may have internal damage and rust.

I am not an expert on accordions and my advice is only from my experience.  However, I will make it a pratice to ask that question from now on.