I'm not sure about Germany, but in the US we only have to pay taxes if we exceed $1000 in sales or over 200 orders per year. Any individual can sell, no need for special permission.
Best bet would be to sign up and see what the Checkout/Android site says. Usually they give good instructions on what to do. You should also look for any FAQ probably. And of course check your relevant government website.
Still, it should be pretty easy just to start. I doubt you need any type of special company papers.Al