How can I increase the sales rate?

I was about to buy that tractor but then...

https://www.cgtrader.com/3d-models/vehicle/other/tractor-case-farmall-a-series
https://grabcad.com/library/farmall-super-a-tractor-1

https://www.cgtrader.com/free-3d-models/vehicle/truck/truck-trailer-97c62ad5-340a-4e6c-9486-2ab25fc57835
https://grabcad.com/library/henred-fruehauf-tri-axle-trailer

https://www.cgtrader.com/3d-models/vehicle/other/tractor-a96706ba-3d02-409b-8cb6-0c292bdf4f4d
https://grabcad.com/library/garden-tractor-legtra001-1

You could at least wrote your own description without copying...

Your designs are beautiful. I really like.
Leave the sales job to time.
It is sold over time.

You have to wait, my friend, it doesn't happen right away.

I would say there is a few factors:

It looks like your last upload was in May, so that's a long time ago. I find uploading more often results in more sales.

Summer results in less sales for some people (North Hemisphere), depending on what things you sell obviously.

Last thing is making sure your items are reaching the right people. Tags, Descriptions and Analytics help a great deal. The more things that can be caught in search engines the higher the chance you will reach the right audience.

I don't think you can compare the last couple of years to any other in history. Sales rise and slump at odd times. Game developers are extending their delivery dates by months to years.
More people are spending time at home and perhaps getting more into hobbies such as 3D printing.

The late summer slump could be that people are planning going back to school so their is the extra cost of that. I guess it depends what market your models are targeting.

With this type of liberalization, the first instinct is to try to launch as many models as possible as quickly as possible. The more models you can use, the more sales you get - right?

Not necessarily.

Even if you have hundreds of models for sale, you won't make a single penny unless they're good enough to make a purchase. Most people willing to spend good money on 3D assets use them professionally, meaning they want to get high-quality work.

While it may be tempting to spread out three or four hour long projects that are "good enough", it's honestly not going to get you anywhere unless someone is willing to buy them.

Instead of focusing on quantity sooner, take your time making your first batch of models as good as possible. Investing the extra time up front will help you build a reputation as a quality modeler. Later, when you've established yourself, you can focus on building up your amount.