It is almost impossible to choose a good domain that is free for registration. It's nothing terrible to buy an already existing domain from resellers it has now become common practice. Well, I would not take this as an important parameter in recognizing the legality of a service.
Yes, of course, if they report one data to BestChange, and have something completely different on the site, it could be considered suspicious.
What can we trust 100% and what not. What will they do incase I get scammed for 5000-10000€?
Is there anything we can trust 100%?
You have to be a little more precise when you talk about unclear things and shady...
Keep in mind that Bestchange is only monitoring where they only collect information and present it in one place, their admin will definitely give you the answers here.