I feel like I've been able to identify with all of the sentiments expressed here at one point of my life or another.
As far as selling items goes, I've done 30-50 sales of individual items from my collection in the last decade. Maybe half of those were LPs and tapes I bought for regular price and then appreciated to an obscene value despite being just ok quality. So those got sold to buy several records of better quality, or at least that's the goal. I don't love contributing to the gross inflation of second hand record prices, but sometimes it feels dumb to take less money for stuff than you can easily get for it. A couple years ago I sold about 40 tapes and 35 records to a record store here in the US, most for $1-$5, if memory serves me. Domestic media mail made it cheap enough to ship for both parties, and I hope he was able to sell a lot of that stuff.
Other than dumping a ton of shitty CDs from adolescence a decade or so ago, I very rarely sell CDs because they always seem to be the most damaged and the least desired so I've now got a lot of CDs that barely play and I don't want but have nothing to really do with them.
Generally my hope is when I sell things off or trade from my collection, whether in bulk or piece by piece, the person who winds up with it likes it more than me. I'd rather a person who really enjoys it have it if I don't care about it.
As far as collecting, well my pace has definitely slowed down. From 2011-2015 I was picking up 250-300 items a year, but as my living expenses have grown, my focus has shifted more towards movies, my free time has diminished and I've bought so many of the things I've wanted, my pace has slowed dramatically. Somewhere between 100-150 records/tapes/CDs a year. I also like to think my tastes have become more refined, even if the range of genres is expanding. One of the things that has always "boosted my numbers" is listening to a lot of electronic music. I buy a lot of new and old techno and drum n bass 12"s every year. It's not a lot of actual music in terms of length but it's a lot of "items". When I get lucky I might pick up 10-20 old d'n'b 12"s for less than $100 so the collection just grows!
I'm lucky that at least right now I can listen to music at work at home and in the car every day. Granted most of the time its digital versions of tapes, LPs or CD rips, all of which I've done myself, but it's still better than nothing. Home is where I get to listen to my tapes and LPs. But my appetite is voracious, typically hearing 4-6 albums a day, at least. And again, listening while working is not the best way to hear music, but it's better than nothing and it lets my memory build up a sort of reserve of what these albums sound like so when I do more attentive home or car listening I can really get into it. I've been doing it this way for about 15+ years and I've liked it so far.
At this point I'd like to think 75% of my collection is good to excellent stuff; 15% is stuff that I'm learning to appreciate or determining it's shit; and 10% is crap I can't part with for sentimental reasons. This all being subjective of course. As was mentioned, I've definitely sold a tape here or there that I wish I hadn't because I didn't give it a fair shot. Shit I've even re-bought some records because I realized their excellence too late. And lastly, I find lately I re-buy records I already own on new formats just to see which format I like best, which feels kinda dumb but what can I say...
All this being said, there is so much good music coming out I still feel excited when I get label and distro announcements, even if I'm a bit slower on the draw and pickier when I order than I was in the past.