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.
craft.dev
(→ Try out Craft Client and Pro on multiple development URLs?).
When you move the license to production you'll see a warning in the CP - you then have the option to transfer the license to the new domain. See here for more info.
Obviously then you'll get the warning in your development environment but that's something you can live with I assume!
Short answer is yes, that is entirely allowed. Great information has already been provided in Tom's answer and Adam's answer.
If you are already certain that you'll be purchasing a Craft Client account, then you won't have to do anything special to get the site working locally. As long as the same license key remains in your site, any purchased upgrades will remain available to you... regardless of whether the site is running locally or on a server!
Not sure how familiar you are with this, but you may want to learn more about multi-environment configurations in Craft. It's incredibly easy to move your website between environments without losing any rights or privileges.
You can run your site locally without any warnings as long as you're using a dev-sounding domain. Read this article... All of the answers you're looking for should be in here:
Any non-public domain will do the job. Just hit Upgrade in your CP then you'll get a option to use it for free as long as you work on a non-public domain so localhost works fine.
When you transfer it to a public domain and you used Craft Pro only options you have to get a license to keep it up like you did on the dev version.