Then how the buyer can combine their own ship if the source file not included?
The parts (wings, engine, cockpit etc) are all split up into individual images, allowing you to import them into the graphic editing software and place them around to form your own ships.
I do see your point of having a PSD with all the parts in it would make things a lot easier (I'm working on that ). I was under the assumption that not everyone will use Photoshop so I didn't include it in the first place.