If you want to do it correctly, I would honestly have to recommend buying a trusted third party solution for both of these things. There are several licensing packages out there for .Net (we use Desaware for our products, although I haven't directly worked with them that much so I couldn't give a detailed recommendation. It seems simple enough for what I've been exposed to). No fuss, no bugs, and the payoff in the long term is huge in terms of time saved.
Third party payment services can handle online credit card transactions for you, and there's always PayPal as well. Google for "how to accept credit card payments on the web" and you'll come up with endless merchant solution providers.
If you'd rather spend the time to build a licensing system yourself, this StackOverflow thread is a pretty good resource on the various aspects to consider.
