Torsten is looking for a convenient system to sell his PDF eBooks on the internet - as soon as the user makes the payment via Credit Card or PayPal or Google Checkout, he should get a link to download the file in his email inbox and the link should expire after a given time.
PDF eBooks - or for that matter any downloadable product including blog templates, Flash presentations, MP3 music, podcasts, video clips, digital photographs, ringtones, software utilities,.. can be sold on the web very easily through a service called PayLoadz Express.
All you have to do is upload the file (that you want to sell) to PayLoadz and they'll immediately give you a link where your site visitors can click and buy the document through PayPal or Google Checkout. It's an extremely smooth transaction.
PayLoadz will also expire the download link after a limited time and also monitors downloading IP addresses to prevent excessive downloading of the file. There's no transaction fee if the monthly limit is $100 and the size of the uploaded files is less than 50 MB.
This Google Video shows a sample transaction - customer makes a purchase and downloads the document: