I'm currently doing contract work for a client in the US and well yeah this is some sort of approach for getting some extra money. Right now I'm studying in Germany and the german tax law applies to me. If the extra money gets over a specific amount per month you'd have to register a traders-license (in case you're selling stuff next to your job, there's the usual income tax and if you get over a even bigger amount you'd have to pay sales tax too) since my passive income approaches didn't pay off a lot yet I'm still in the boundaries where I don't have to worry about that.