I buy music CD’s all the time, new and used and for me the big problem with buying collections is
1) What to do with copies of the titles you already own
2) What to do with said copies if there is no jewel case/artwork
3) Can the collection be broken down by genre or does it have to taken as a whole
4) Where is the collection and is it worth the shipping cost
5) Is the price realistic for what is being offered
Some people are very diligent with their collections and go as far as to catalog each title via Discogs or on spreadsheets. I think in your case you would have very good luck selling the Mofi, SACD and HDCD titles separately or grouped by artist as there seems to be demand for these amongst collectors when the price is right.
As for CD’s with jewel cases/artwork there is some value there but it doesn’t seem to ever equate to large amounts when considering the price per unit. Some local stores in my area will pay (when they pay) as little as $0.50 and up to $2.00 depending on the disk but most of the time you’ll see a larger amount working it as trade in value/credit.
The discs without jewels or sleeves/booklets are pretty much worthless when it comes to in store trade’s although some folks who don’t care may be interested in buying for a much lower price.
IME with rippers/servers and discs I would say please keep the discs that are important to you even after ripping/backing up. Even with multiple drives things can go wrong and at this point you already own the media so unless you just don’t want it around the only penalty is storage.
Streaming is nice but I still prefer discs! Be confident when selling/purging that you won’t want to repurchase if you change your mind at a later point.
Hope some/any or all of this helps and like I said I’m always looking for music. If you’d like to send me a note describing what you have I’d be interested.