I have Industry Standard 3D Models...But no sales...Reason please?

Reasons:
mainly (customer) demand
+ (extreme amount of) competition

The sales you got with these assets (considering amount and quality) is normal, so there is nothing wrong.

3D online industry has changed dramatically in recent few years. It is no more like in 2010 - selling 3D models like gold mine and easy money.
It is no more sufficient to do good model of interior, chair or TV. Market is fully saturated by such models of premium quality and low priced.
If you want to succeed in 2020, 2021 or 2022 you must do something challenging, unique, original, complex, not the same thing like 100 designers before you.
Do research of market before modelling ask yourself are you able to do something better if it is already included in market?

I think your technique is outdated. I watch your castle, and it's look nice from the far, but if you watch closely you will see most of the UV-s are incorrect some of them are rotated by 90 degrees. The stairs have also strange UV-s. You need to invest more work in texturing, and UV. If you want make a proper castle you need an absolute different workflow. You need to start from the pieces, make a wall, texture is correctly, make a roof texture it correctly, make a door..., make a stairs..., ect. After you have the pieces you can build the castle. And of course there are another techniques, but I think this is the easiest. Your another modells(gun, has no textures, the mesh is useless) also has serious problems. But I can recommend you a very nice tutorial for props modelling: artstation.com/artwork/D5kXE0 you will find the gumroad, or google drive ling in the description

Hi there,

Thanks for reaching out.

To increase your sales, you need to promote your 3D assets.

First, generate nice previews of the model and use them to announce new models on your social media accounts, in design forums (not only on CGTrader) and in any portfolio sites.
Always use a direct link to your model page to increase your model's ranking in search engines to have a stronger position in future sales.

As you model a new asset, make sure you film or screenshot the process and upload them as tutorials or ‘how to’ posts with links to the finished project available for sale. Tutorials also help you gain a following and grow your fan base as a 3D artist.

You can promote models directly by generating a unique coupon code for your chosen discount on your models.

Add links to your asset pages in portfolio sites, like Artstation, Behance, DeviantArt, Polycount, Pinterest and others. If you have followers on these sites, they should get notified if you post a link anywhere on those sites.

Use a small banner to advertise your shop on CGTrader in any design related page (like a signature with a link in design forums, etc.).

How can I encourage people to download some of my models?

You should always have a simple model available for free download to let potential buyers see your skills.

Be sure to use advertising tips in this section, and share your works on design sites and social media to increase your exposure. Generate more reputation and have a good response ratio and response time, to let customers know that you will respond quickly if they need help or support with the model they bought from you.

Kindly,
Agota from CGTrader