Christian, please click on your avatar & go to your Profile, where you can add your first name just one time in your signature.
All of us were anxious, at one time, to buy our first Japanese sword, so we understand what you're feeling. So, please trust us when we tell you to stay away from eBay, like the plague! After you've spent a year or three studying, & handled some blades, then you can go back, because you'll be looking with different eyes.
The NMB For Sale section is a good place to consider for your first blade, but there are other long-time sellers. I love browsing therough Ed Marshall's yakiba.com, & I've bought a number of his well-documented swords. You can learn a lot by just looking at his photography.
One other piece of advice. You do NOT have to start with a katana! One little secret is that a well-made tanto will sell for just as much as a well-made katana.