Earlier quoted context omitted.
Nice to see that there's no limit where I live which is Norway. Hopefully, there are no plans to introduce one.
I am confused by your post. What are some good reasons to allow cash payments above 3000 EUR?
Cash is, after all, the only instant and feeless transaction method. Why would I want to pay % fee or wait up to several days for a transaction like that?
Imagine I bought the car on Friday afternoon. The seller would get the money earliest on Monday. Why would the seller simply trust me? So how would I buy a car on a Friday afternoon otherwise?