“Listing violates Duplicate Listing policy” means that you have the very same item already listed on eBay. eBay doesn’t like that and therefore denies your listing.
This happened with more than 200 listings; in fact, it happened with every book I listed.
I understand that either eBay or GarageSale believes they are already listed, but that is not the case with even even one of them. I know because I spent more than an hour double checking each and every one which got the error message against the ones I have listed.
As a test, I listed 4 of them manually and directly through the eBay interface, and I had no problems. Not sure what that’s about.
Can you explain this message:
"Product You Are Trying to List Against Is Not Available for Selling"