If you wish to presell your book you can use our Update service to deliver the download link when the book is ready. Our help page for the Update service is here:
The basic idea is that you set up a product for the pre-order now and sell it for any amount you like. You don't have to deliver anything, but it may be a good idea to give a download of a sample to tide your advance buyers over until the book is ready. We will compile a mailing list of everyone who buys this product that also agrees to further contact.
If you are using PayPal Payments Standard to sell your product, all of your buyers are automatically added to that list, so you may want to use that payment service specifically. The other payment options, including PayPal Payments Pro, include an opt-in box that must be checked in order to add the buyer to that list. If you use one of these, be sure to tell your buyers to agree to that if they want to receive their update later.
When you are ready to release the book you can either just upload the full file to the original product or even create a new one. Either way you will have the option to send out an update for the full book to the buyers of the pre-order product. There is a small fee involved, explained on the help page above, and we will send out a download link to every single person on that list.
You asked about PDF stamping previously, so it is important to note that files sent out with an Update cannot be stamped for technical reasons.