If you made it and it is your own original work then you automatically have copyright on it.
Assuming we're talking about this thread, this is to quite right as it applies (generally but not necessarily 100%) to ORIGINAL work - not replicated work.
Even though you might have made a ATAT from scratch you don't own the right to selling the model because the original concept and design is not yours to sell - its licensed. You might well be selling a
great item but you haven't had to design it from scratch AND you haven't had to incur the cost and headaches of creating a brand around it first. In reality this is no different from the Chinese fake Apple, McDonalds or IKEA stores. Although they might sell items that are 100% as good as the originals (for argument's sake), they're trading off an existing designs and brands that are not theirs.
The only time this changes is generally when a trademark/patent license expires as we're seeing with the manufacture of the ST helmets and R2 units at Shepperton.
Generally the studios turn a blind eye if you make a
single item and sell that as your original art - even a model like an ATAT - but they can get seriously grumpy if you decide to make copies of your ATAT and sell multiple reproductions to collectors because to do that you need to be a licensed vendor.
Obviously I'm speaking about things in very broad terms and there can be exceptions but generally, the core issue remains the same…
Regards
MARK