‘Not property released’ means that the author of the item you’ve purchased may not be the owner of the intellectual property rights in a real-world product or trademark that appears in the item. If an item contains a real-world product or trademark (or any other real object or building), the original owner of copyright, trademark or other rights in that real-world thing may not have cleared the item. If an item does not contain a real world product, trademark or other real object, then this notice doesn’t apply.
So if you want to use the item for your project, you’ll need to consider how you’ll be using the item. If it’s for a non-editorial purpose you’ll need to get clearance from the original rights owner. That’s because there is no connection between the original rights owner and 3DExport or the author. The original rights owner also does not endorse the item, the author or 3DExport.
For example, the author has created a 3D model of a Mercedes car. The car is a real-world product which the author did not create. Although the author created and owns the 3D model, they don’t own any intellectual property rights in the real car itself, Mercedes does. So it’s OK to use the 3D model for editorial purposes but if you want to use that 3D model for other purposes, you’ll need to contact Mercedes to get permission.