Shaikh Zayd al-Madkhali rahimahullah was asked:
What is the ruling on buying and selling books of knowledge of authors whom we do not know their condition other than (that) they are not known for the innovation of partisanship which is blameworthy and nor other than it. In their books are benefits that a student of knowledge needs. Likewise in regards to their tapes? Please give us a verdict, may Allah reward you.
There is no harm in buying beneficial books from authors who are not known to be innovators and it is not obligatory to prevent buying it. Whoever wants to know the condition of its author, then let him ask those other than him of the people of knowledge. If he does not find one to ask, then let him read what is in it. If he finds good then let him praise Allah and let him be directed to owning it and benefiting from what is in it, especially when there is a need for it. Whoever finds other than that, rather he finds harm in it from recording innovation or grave mistakes, then let him warn from these books. Let him warn others from it and earn a great reward and protect himself and others from a dangerous evil.
The same is said about the tapes of these people. It is not said there is a separation when it comes to their books [from that of their tapes]. It is not necessary to divide between books of the past and the contemporary books. The affair is like it was mentioned. There is a difference between the books of the past and the contemporary books only when there is found what breaches the Aqeedah and academic methodology. Then it is allowed to buy the books of the earlier Scholars like the books of Ibn Hajr, an-Nawawi, al-Qurtubi, Shawkaani, Abu Hanifa and those who agree with him from his followers. This is because Scholars, even though they have mistakes in Aqeedah, (it is the case that) their books have legislated knowledge which the people cannot suffice without. Unlike the contemporary books. It is not allowed to own them if there is innovation and misguidance in it and something of beneficial knowledge. It is possible to do without it because if it is allowed to own them, it will become a trial for the students of knowledge due to what is in it of innovation and misguidance becoming truth. This is whether their opposition is in Aqeedah or manhaj. With Allah is Tawfeeq”
[Ajwibatul-Athariyyah ‘ala Masaail al-Manhajiyyah pg. 65]
Faisal Ibn Abdul Qaadir Ibn Hassan