In most cases, it is not illegal to sell other peoples 3D prints, as long as the prints do not infringe on any copyright laws. Many 3D printed objects are usually made from designs that were found online or created by someone else, and in these cases it is generally allowed to legally resell those items without needing permission of the original creator. As a precaution though, you should always check with the artist or designer before selling an item that was designed by another person. If your item does contain copyrighted material then you must obtain permission from the copyright owner before reselling their work in order to avoid potential legal issues.