decreasing Sales...is it happening to everyone???

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I agree, CGT sales dropped last months, while other large competitor site sells my models in a usual manner.
I suppose something happen to the search engine algorithm of Google perhaps, as I noticed that if I search for my model I get results from that other site rather than CGT.

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.

Not for my part. Normally my sales decrease during summer, but not this year for some reason. I hope it is an ongoing trend and not just an incident.

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.