Continuing to use it on a regular basis is prohibited since it is not yours, and it is explained in the rulings that it is forbidden to use afi' for a mitzvah in other people's books on a regular basis, and also if he continues to use an object there is a fear of forgiveness and waiving the mistake of the taker is a mistake, but to use it randomly in a way that does not intend to forgive it , it is possible that it is legally permissible to use someone else's book for the purpose of a random mitzvah that the arbitrators decided is permissible, and in this there is reason to say that waiving the take is not a mistake, since even if it was not the buyer's at all he would be allowed to use it.
And while the books that are up for sale are certainly being taken care of so that they will not be used, but a damaged book that is about to be put away, and it has already been used sparingly until now, it does not appear that it is taken care of if it is used randomly and carefully.
Sources: Regarding the use of books, according to the Rama'a Okh Yad and Maga and the Sh'nav that Afi' is prohibited randomly, and see Bm 20 18b, and in the opinion of Nuta 20 Hagarhak Dahaidna is allowed randomly, and so is the prevailing custom, And the KMSH in the OT there (even though the main point of the law is implausible to her) and in the Likuti of the Rih that this is the custom today, and it is possible that today the books are more resistant to random use, and some wrote a model because the books are cheap today and less careful about them, "A C. Ksg S. K. Ka, and O. H. Hom. 17 regarding books in Bahkanas, except that it does not pertain to our matter).
In the case of another user who was shocked by the Maqh that invalidates the Maqh's claim is a mistake, see HOM Relav, 3.
מק"ט התשובה הוא: 2503 והקישור הישיר של התשובה הוא: shchiche.com/2503
