I'm sorry, but those "expert sellers" have given you bad information. By any chance, did they sell you this product, with "master resale rights" included? That's an old scam, there's no such thing as master resale rights. Unless you created the item yourself, you don't have the right to sell it. The creator is protected by copyright or other digital rights, and the item can't be reproduced (which is what selling the digital file is doing).
Classified Ads are only available in a limited number of specific categories, and if your item doesn't fit into one of those categories, then you can't list it in a classified ad. The categories don't include any digital item that can be printed, whether it's paper printing or 3D printing.
https://www.ebay.com/help/selling/listings/listing-tips/selling-classified-ads?id=4167
https://www.ebay.com/help/selling/fees-credits-invoices/selling-fees?id=4822
