Old harmonicas come in many shapes and sizes. They also come with many different types of writing and other types of identification on the cover plates and the box.Try to describe what you see.
For a collector the following information is critical in making an informed decision.
Maker ---  (if known ) such as Hohner, Koch, Seydel etc.
Size-----   Length and width  in inches and/ or cm.
Number of holes --- and if they are double holes.
Name of harmonica---  (if known) such as Marine Band,Concert,Echo etc.
Country of origin----- ( if known) Germany, Austria, Japan etc.
Condition----- do all reeds play? any rust?any broken hole spacers?any cracked wood? etc
Condition of the  box  can be very important in determining the value----
All these are helpful but the best tools are good clear pictures. We need pictures of both sides of the harp,pictures of the playing surface,and pictures of the inside and outside of the box. If that's too many pictures then a very clear description will do.Fuzzy pictures or ones that are to small can lead to problems with the buyer.A description of any labels on the box is helpful.
Also a clear outline of your shipping and payment policies are always needed. Charging  inflated prices for shipping and packing are a real turn off.. Most harmonicas can be shipped standard USPS for under $6.00
Always offer insurance. Many old harmonicas are hard to replace regardless of cost.Please pack them well.
Thanks for your consideration H Crain president of Harmonica Collectors International