Hi!
If I were you, I would simply contact the designer who designed the model you are going to build. If they allow you to sell the physical object, there's no problem as far as I'm concerned, because it's partly your work too!
By the way, I can ask you to have a look at my designs, all 3D printable ones are good in quality and free to download. If you are interested in printing and selling some of my designs (or just want to download them and print them for personal use) please let me know - contact with costumers is always great! :D
Best Regards,
Christoph