If you don't mind shipping them the book(s):
Internet Archive, Attn: Book Donations, 300 Funston Avenue, San Francisco, CA 94118
If you have a large donation of books, please call first to make arrangements: (415) 561-6767
"(2) No reproduction, distribution, display, or performance is authorized under this subsection if— (A) the work is subject to normal commercial exploitation; (B) a copy or phonorecord of the work can be obtained at a reasonable price"
I'd imagine that "normal commercial exploitation" would generally mean an item that is currently in print, although I could see other theoretical cases this would cover (for example, the work is not technically being printed anymore but there is a huge stock of existing volumes on the market).
I don't think reselling rare volumes would fall under this category - that could be "commercial exploitation", but certainly not "normal".
If individuals have private scans of rare books (1923 - 1941), how should these scans be delivered to Internet Archive for hosting?
Does "not being sold" mean out of print? Used, out-of-print books may still be sold. In fact, that's sometimes the only way to find a (possibly very expensive) copy for scanning and preservation.