Selling a 3D-printed duck is legal when two things are true. You have a commercial right to the file, and your listing tells the truth about what the object is.
Start with the file, not the shop
Before you photograph anything, read the license for that file. If it is personal use, the print is for you. If it is an active Duck Heroes commercial license, you may sell the physical duck. You may not sell the file.
The Duck Heroes catalog is not open yet. Do not pull a lookalike from a free site and assume the rules match. Another designer’s file has another designer’s terms.
Say what the object is
A Duck Heroes model is an original duck sculpture of a theme. The film, game, or character name is not yours to license.
In a listing, describe the object you made: a 3D-printed collectible duck, the theme in ordinary words, the size, the material, and that it is a fan-made interpretation. Do not write “official,” “licensed by,” or the studio’s logo. Do not use key art you do not own.
Credit to the designer is welcome. Pretending you are the designer is not.
What you can change
You can paint it, pick another filament color, scale it, and add a magnet, stand, or keyring. Those are physical changes. If you remodel the mesh, you still cannot publish that new digital file.
Where the sales can happen
With an active commercial license, ordinary channels are fine: your own shop, marketplaces, craft fairs, conventions, and small wholesale. There is no country limit on shipping the finished duck.
Keep the STL off the packing slip, off the thank-you insert, and off the customer’s download page.
When the license stops
Cancel anytime, with no fee, once a paid license exists. Selling rights run through the end of the period you paid. Then you have 30 days to sell ducks you already printed. After that, take the listings down unless the license is active again. Files you downloaded stay available for personal printing.
Membership is not on sale on this site today, so there is no plan to cancel yet. The refund policy explains the rule for when there is.
Pair this with pricing before you pick a number, and with photos before you publish a listing.