Thank you sorcha so much for the information about haushueter.org! And the rest of you for supporting/clarifying comments. I do understand completely that this idea of dog sitting and letting a stranger in your home may sound weird at first. But I believe also that this type of activity will be more and more common in the future. And sorcha, you picked up the very problem I'm dealing with here. That is, there is no official site that would bring the home owners and sitters together in Berlin and that I could register to. It would be more secure to me as well (!!).
However, I have traveled before and I have learned to stay away from trouble (one year in the U.S. for example). I do not take uncontrollable risks. Let's say that somebody offers to have me as a dog sitter. In this case I would contact this person by phone. And/Or, if possible, I would ask them to register to Trusted Housesitters. For home owners there is a free 5 months' trial (by the way:). I understand that on this site they have a contract that is filled between both parties involved. That increases the amount of security. Moreover, the home owners get to give a grade to the home sitter after the assignment. At least I want to have five stars (!!).
About my trustworthyness, what can I say? Well, I have worked for the local police in 2006 and before taking on that job they made a security check on me. Which, of course, was clear (I wouldn't have gotten the job, if it wasn't). I can send a copy of the testimonial if needed. It is in Finnish but there are all the logos and so on. I also made an internship for the National Bureau of Investigation (it is the FBI of Finland) to help to solve economic crime. So, I tend to consider myself more of a good guy, rather than bad.
And for those who want to find me in Trusted Housesitters pls go to trustedhousesitters -> Find a House sitter. There you will find searh boxes, one of them saying 'current location'. Choose 'Finland' -> Refine search. There you will find four listings, one of which is mine.
So, how does this sound? Do you still think I'm weird ?