A slide is a positive image with the color dyed into the emulsion during processing. Kodachromes look like a positive b&w image before the dye process. Unlike color print film (or black and white film) there is no negative. A slide is the entire product in a tiny package. This is why they where so popular as there was no negative or print to keep around.So beings I know nothing about the slide making process, I'm guessing there is a negitive involved in making a slide. And if a slide is made from the negitive, it's an original. But if it's a copy from another slide, it's a duplicate. Is that correct?
OK, so if I do go the e-bay route and sell by the slide, there is no way I can say for sure if they are original or not. What if a guy could secure the negitives? Would that help?
To make a duplicate slide you basically take a picture of the original slide using slide film (ironically the image of the image is an original). This process results in a loss of quality from the original slide. A collector will spot a duplicate slide fairly easy though I have seen some extremely high quality duplicate slides that are really difficult to tell the difference even when the original is shown next to it.
If you sell them on Ebay and you do not know if they are a duplicate or not just say that in your listing its up to the buyer to make the call on whether they want to bid or not. Now it would be save to say that any slide with a Kodak/Qualux processing stamp and indicates its a Kodachrome is an original.
What makes a slide worth more than another is the subject, the composition, lighting and condition of the slide (damaged mounts, mold, dirt, scratches, etc will kill the value) plus the whim of the buyer. A good scan image also makes a difference when selling them. If you don't have a good slide scanner if maybe beneficial to buy one or know someone who will scan them for you (I've been know to do this for others). A medium quality scanner can cost between 200-500 dollars while a top of the line Nikon scanner can be well over 1500 dollars. Used ones on Ebay can be found at a good price if you are patient.