Can you legally sell 3D prints?
Updated
The short answer
You can legally sell 3D prints of models you designed yourself. A model you downloaded carries the licence its designer attached, and many of those forbid commercial use, so check every file you did not make. Characters, logos and brands stay protected even when the model is free.
Those are two separate questions, and you need a yes to both.
The licence on a downloaded model
Every model on a sharing site comes with terms. Many are non-commercial, which means you may print it for yourself but not sell the print.
Some allow commercial use with credit. Some allow it freely. Some say nothing at all, which you should read as no.
If you want to sell prints of a model you did not make, ask the designer. Written permission is quick to get and settles it.
Brands and characters are a separate problem
A free model of a film character is still a film character. Trademark and copyright sit with the studio, not with the person who modelled it.
Fan art sells until somebody notices. Then the listing comes down, and sometimes a letter follows.
What is safe to sell
Models you designed. Models in the public domain. Models under a licence that plainly permits commercial use.
Original designs are also the only ones where you can set your own price, because nobody else is selling the identical print.
Keep a record
For every item you sell, note where the model came from and what its licence says.
That record takes a minute per product, and it is the thing that answers a complaint months later.