Cassettes generally have no market value now. Even Cassette walkman devices are hard to find.
The Lp's you list are all fairly common and realtively easy to find on vinyl.
You raise an interesting question: What to do with vinyl? I discovered E-bay when itv was new and was able to sell off a large chunk of the Lp duplicates and triplicates I had accumulated over the years. Even on Lp the London cast of BARNUM brought a large number of bids. But within a few years intereset dwindled.
Libraries have switched to CDs and rarely accept LP donations.
The library at Goodspeed used to accept donations but wound up with too many d8uplicates. (Also, Goodspeed took over publishing of SHOW MUSIC magazine and then suddenly stopped publishing it and refused to return the materials to the original owner effectively putting a valuable publication out of business. So naturally I do not promote any donations - money or materials - to Goodspeed any more.)
Surprisingly there is no official archive of the American Musical Theatre. The R&H Library at Lincoln Center still has Lp's but I imagine they are well stocked with Annies and Funny Girls.
I have a huge collection of theatre books and recordings and I have instructed my family not to worry about keeping any of it once I am gone. The pleasure of collecting this material is in the now and for me personally. In 10 years I cannot foresee it having any significant resale value.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com