@charles70
I often ask myself this question, especially for the whole collection of erotic books, of female nude photography books, of all the photos I have taken (but many have been published on books and exhibitions all over the world, so this part is free of worries) and all the erotic material for the various games.
There are dealers and shops dealing in all kinds of secondhand materials. Erotic materials are in high demand, not by all, but many will be able to refer you to those who do buy and accept erotic material.
Please, please, please do not destroy your erotic collections, especially of books, magazines, drawings, photos and especially of very personalized creations like diaries, journals, sketch books or such. There are collectors who really pay substantial sums for such material, no judgements made and who cherish and keep such material in good care.
You can find some of these dealers/collectors on the web. I also know of some located in Brussels and Paris (with shops) and undoubtedly there must be some in larger cities like Rome as well. It is difficult to find true erotic material nowadays since many astute people with a nose for business have figured out that such material often makes for good publishable books. Nothing better than to come upon an anonymous collection of private photos or drawings of an intimate nature from some 40 or 50 years ago.
Recently I know of a series of photos from the 1930s of the Parisian art student's annual ball and parade (Bal des Quartz'Arts where many students dressed up or went nude) was sold for some thousands of euros. Out of my league unfortunately. I surmise they will be used in an upcoming book publication some time soon.
And if you cannot find any dealer interested in buying, one can always donate interesting collections to for instance a Kinsey Institute or similar institutions. These at least keep and preserve material for posterity.