Craft license and develop a website locally? Is there any problem with buying a Craft Client account, working on the website locally (on my pc), and when it´s done move it to the server?

Is this permitted by the license?
Sebastian, you can always work locally "for free", using "craft.dev" as host name.

Check the FAQs here at the bottom of the pricing page https://buildwithcraft.com/pricing

MAMP PRO makes it really easy to setup custom host names, btw.