Is it possible for a layperson to spray walls and ceilings? Experiences?

And regarding the color... would DIY store paint (Alpina) also work, or does it have to be specifically airless paint? I assume that you have to dilute any paint...

Use high-quality paint, you won't want anything else afterwards. Good paint does not need to be diluted.

I can't deal with this attitude.

Buy the device, use it, and then return it.

You are reading too much into it.

We wanted to paint the whole house and bought materials for it. The salesperson tried to sell it to us but said he himself had no experience with it. Therefore, he offered us on his own to test it and return it if we didn’t like it. After the hallway, we did that and painted the rest in the traditional way. So nothing about a bad customer, poor trade.

Okay, if it was agreed upon, the situation is completely different. Then that's fine.

Every good paint can be sprayed airless. Just take a look at the bucket or data sheet. Usually, something needs to be diluted. I have an airless device that I no longer need. Primer and paint are also still available.