Then maybe try ebay? You can list stuff for free on there.
I may be a bit odd, but some of the non-fiction books I have bought on ebay (because they are literally not available anywhere else) have been:
"dated self-help/DIY/decor/travel type books"
Reader's Digest Book of Home Improvements, 1977 (I think it cost around £10)
The Road to India: Guide to the Overland Routes to the East - John Prendergast 1977 (I paid around £15, today it is around £30)
Reader's Digest Folklore, Myths and Legends of Britain, First Edition 1973 (I paid around £30 but good copies are going for £80 plus these days)
.
"...memoirs from people who died decades ago"
These can also be worth a little bit of money. Some time ago I bought a first US edition of The Small Woman on ebay for £10 (it was published first in the UK in the same year and then a few months later in the US).
Nowadays, different editions are selling on ebay for around £15-£20.
For context, this book is the biography of a young British woman who went out to China as a missionary in the 1930s.
The book was then later made in to a film in 1958 called 'The Inn of the Sixth Happiness' starring Ingrid Bergman.
https://www.mumsnet.com/talk/womens_rights/5469562-theattagirls-on-twitter-the-true-story-behind-the-1958-film-inn-of-the-sixth-happiness
So, have a look on ebay and see if other people are listing the books you have, for sale.
If so, then there is likely a market for them.