domain Internetski.com - croatian for 'internet'

hi, it doesn't mean 'internet' in croatian. It doesn't sound good as a name at all, incomplete. Not even a noun in this case.

Croatian internet is same word as english word - internet

Native Croatian speaker here, I would say it is decent.
However, it does not mean "internet", it is descriptive adjective.
If you are developing it you can use it for eg. forum (internetski forum) or portal (internetski portal).

ok... I'm native speaker too and can't see any value and sense in this domain, but I'm glad OP found the buyer. Just wow. Even croatian extension is free to reg.

There are examples of similar adjective domains on our ccTLD like: dubrovacki.hr, jutarnji.hr, poslovni.hr, studentski.hr, vecernji.hr and znameniti.hr, so I would say it has a value.

These are mostly well known newspapers and it's their shorten version which we use in everyday speech. That makes sense to me, but 'internetski' is incomplete without noun following it.
But hey, if you see value or you have an idea to develop it - why not. Cheers!