First off, congrats on finding so many! The thing with ebay is, if you don't stand out, you won't sell.
There are a few kinds of listings I typically find on ebay for canister shot.
1. Round iron or steel balls that are not canister shot. These often get avoided
2. Real canister shot, but these are rusty/uncleaned. These sell, but they are also the most common thing out there. If you want to sell rusted canister, and beat the competition, you have to lower your prices by a dollar or two just to be noticed. Either that, or make it free shipping and work the cost of shipping into the price. It's a fact that people will buy items that say "Free Shipping, over one that has shipping, even though they're the same price. It's a psychological thing that we do I suppose.
3. Clean, authentic canister balls. These are tough to find, and if you take the time to clean them, it will be very much worth it. Think of it: Would you buy an artillery shell, say a Read, or Parrot, if it was fresh out of the ground, rusted, dirty, etc. or would you buy a nice black one that has been cleaned and preserved? 9/10, folks will buy the clean stuff instead of cleaning it for themselves. Now I on the other hand buy dirty stuff so I can enjoy cleaning it. Not everyone is like me though.
In short, you have to stand out. Try to clean them up. Take as good a picture as you can of them, make sure there is good lighting, pics aren't blurry, etc. Show a size reference, like a quarter for example. If all else fails, try to undercut the prices of others by a dollar or two, just so that cheapskates see them first. STATE THE PROVENANCE. You don't have to tell everyone where your secret site is, but if you make sure to state that you found them near a certain town, battlefield, landmark, etc. people like a good story.
If ebay is not your thing, try selling them to dealers like Virginia Relics. Depending on provenance, Andy Keyser from Gettysburg Relics likes that sorta thing.
If you need any more questions answered, I'm here until the weekend. Good luck!