Praenumeration was a common business practice in the 18th Century book trade in Germany.
The publisher offered to sell a book that was planned but had not yet been printed, usually at a discount, so as to cover their costs in advance.
The business practice was particularly common with magazines, helping to determine in advance how many subscribers there would be.

For example, when Johann Heinrich Zedler

Johann Heinrich Zedler

Johann Heinrich Zedler was a bookseller and publisher. His most important achievement was the creation of a German encyclopedia, the Grosses Universal-Lexicon ,...

 decided to issue a collection of works of Martin Luther

Martin Luther

Martin Luther was a German priest, professor of theology and iconic figure of the Protestant Reformation. He strongly disputed the claim that freedom from God's punishment for sin could be purchased with money. He confronted indulgence salesman Johann Tetzel with his Ninety-Five Theses in 1517...

 he advertised that the book would be for sale through Praenumeration at the Leipzig

Leipzig

Leipzig Leipzig has always been a trade city, situated during the time of the Holy Roman Empire at the intersection of the Via Regia and Via Imperii, two important trade routes. At one time, Leipzig was one of the major European centres of learning and culture in fields such as music and publishing...

Easter Fair in 1728, with the first volume to be available at the following Michaelmas Fair in early October.
When the eleventh and final volume was issued in 1733, Zedler found himself in difficulty. He had been using Praenumeration payments for the future volumes to pay the bills for previous volumes, and now the last bills were due with no future payments to cover them.