How quickly can you sell new goods on Ebay?

Are there many buyers there? Or do you have to wait a long time?

eg laptops or cell phones

Depends on the price. The cheaper, the faster. Otherwise, it can go away after 3 days or after 3 months. Difficult to say in general.

It always depends on the price and the model.

First you need a trade license for such a trade. And if you ask here, you should leave it at that.

It was about retail sales.

You generally ask about cell phones and laptops. That certainly doesn't sound like retail sales.

It depends on how quickly you apply for / receive your business license.

Here is a rough overview of the duties and rights of commercial sellers:

https://pages.ebay.de/rechtsportal/gewerbliche_verkaeufer.html

Follow the individual links.