My advice is put all your renders on a white background, showcase your products like a showroom floor. You are losing lots of detail by putting it on a black background. It also looks very unprofessional in my opinion. As a professional with more than a decade of experience when I see any render on a pure black background I instinctively recognize junior work which makes me instantly not buy since I know the problems I'm going to get.
You say in the description this is made in blender and you dont provide the blender format, only obj, stl, fbx, dae. Why dont you provide the original format (blender)?
Your model exceeds 41,000 polygons for a simple model like this which is too much for a weapon model of this caliber.
You should mention the texture resolutions and bit depths per texture if you intend on this being for AR/game purposes. Customers who are buying weapons for games need to know this information.
You mention PBR textures, what textures are you using? Do you include occlusion? I think you need to add more to your descriptions since its very difficult to see what you get in this purchase.