Why did my domain receive a penalty?

Hi!
I've received an automatic penalty 1-2 months ago for my german domain "kostuemkiste.com". My rankings dropped hard, but the domain is still indexed. Linkbuilding is almost non-existent so I highly doubt some offpage could've been the cause.

Anyway, my rankings havent recovered yet even though Ive tried adding more content to my product-categories and cleaning it up a bit in general.

Anyone able to identify a clear reason for the penalty? Not asking for a complete analysis, but maybe someone sees a clear reason quite easily and I don't.

Domain is german and the URL: kostuemkiste.com

I'm new so I'm not quite sure if this is the right place to ask something like that - or even if BHW does have a place for my question.
Anyway, thanks in advance to anyone kind enough to have a look.

Duplicate Content?

oni365 said:

Duplicate Content?

Click to expand...

The content on the product pages is duplicate as it's just imported from Amazon, but these posts are all on noindex.
Everything else is unique.

Repetitive keywords in the domain.
Copy and paste the copied content

The content is not worth reading
Content full of dirt so it is not useful to others
Your website reported as spam.
Backlink spam in large quantities with repetitive keywords and the same

Jobdollarr said:

Repetitive keywords in the domain.
Copy and paste the copied content

The content is not worth reading
Content full of dirt so it is not useful to others
Your website reported as spam.
Backlink spam in large quantities with repetitive keywords and the same

Click to expand...

Are my URLs meant with repetitive keywords in the domain?
What do you mean exactly with content is full of dirt?
How exactly can I check whether my domain has been reported as spam and how do I find out who by?

Thanks for your help so far.

20% of keywords in the website are suggested.

Not planted keywords to the content of black or white, old techniques

Interesting content and research is useless without previous content, and content in the search contains lots of bounce rate.

Useful content usually contain many links and confuse users