Here's an interesting teshuva from one of those horaah type collections. The question is, is it permitted to sell Mendelssohn's books?
In fact, the response only concerns his Chumash. The question is, an antiquarian book seller noticed that he had acquired a very nice Chumash which upon inspection proved to be the infamous Netivot Shalom edition by Moses Mendelssohn. Is it permitted to sell it, or not permitted either because of Lifnei Iver ("do not place a stumbling block before the blind," i.e., the buyer) or perhaps it is in the category of things which are prohibited to sell.
The reply is that since it is well known how terrible the man was, and what came about because of him, amalek, etc. of course it's terrible. However, we cannot forget that many great rabbis did not discern the impurity in it. The story concerning Maharam Shick and the Chasam Sofer is brought, where the rabbi and his student disagreed about the intrinsic evil of it. Therefore the bottom line is that we simply don't have the power to judge it literally as a heretical work according to all its laws, even though undoubtedly one must not read it.
As for the question of Lifnei Iver - which could still apply - this would certainly seem to apply. However, in the special circumstances here, where we are talking about selling and buying antique books, there is a leniency because many people buy old books without any intention of using them. He checked with Rabbi Shmuel Eliezer Stern and Rabbi Nosson Gestetner and both concurred, that to sell such books to collectors is permitted.
Indeed, many must have wondered such things when browsing the shelves of apparently piously owned book stores. Of course on the one hand business is business, and on the other hand not every apparently pious book seller has any interest in appointing himself the sefer police. And if he did, en ledavar sof. I was reminded of a recent interview in Mishpacha Magazein with the proprietor of the famed Biegeleisen Fine Emporium of Hebraic Literature in Boro Park. The owner, Shloime Biegeleisen, seemed amused/ half-annoyed by the interview. It seemed like he didn't understand why he was being interviewed , not considering himself a celebrity or a legend, and was used to people who knew seforim, not people who were trying to extract a juicy conversation for a magazine for the mass market. One of the questions he was asked was something about how does he deal with questionable books. A dumb question - my apologies to the interviewer - for a mocher seforim. So he replied, that then only book he wold never stock is one which attacks Belz (he schtams from Belz) - a reply which seems to have been taken seriously.
See these two interesting posts on Chabad Revisited on "What Mendelssohn Did Wrong." I and II.