The overwhelming majority probably ended up in the landfill. At the first station I worked at, I saved a few LPs of various pre-recorded programs we ran there, mostly on weekend mornings (Powerline with Brother John, Rick Dees' Weekly Top 40, etc) and then a few CDs from when they switched from LPs to that format, but the overwhelming majority went into the dumpster. Long-form programming that came in on reel usually got tossed into a box until it got full, a few were erased and reused for production, but the majority got thrown away.
At a later station I worked at, the transmitters were adjacent to the air studios. After the building and transmitters were damaged, they decided to move them to the basement which was more secure and entirely encased in concrete. That meant clearing out the basement space and the amount of stuff in storage was pretty amazing as the station had been on the air in one fashion or another since the early 1950s and as one generation of equipment was replaced by the next or they updated studios and equipment, all the stuff they didn't know what to do with but didn't want to throw away ended up in there. It was like a multi-generational time capsule of sorts: There were old RCA on-air consoles, loads of old mics, cartridges and cartridge players of all shapes and sizes, turntables of various generations, carts and reels from various networks and agencies, rudimentary automation systems, old mutitrack reel to reel machines on wheels once used for production, mixers designed to be used for live remotes when they were using TELCO lines for those and countless old manuals, catalogs, schematics and old stationary and give aways with past logos on them, etc. To some, that would seem like a treasure trove, but in their case it was seen as "old junk" that had to go.
They had a large dumpster that was open on one end backed up as close as they could get it to the door, and most all that stuff I mentioned, found its way inside to be hauled away. Several reels of the type of music you've searched for on eBay also made their way into the dump. The station had a contract engineer who had no real connection to the station and didn't really care about any of that stuff and he just wanted to make a spot for his transmitters. Most of the staff cleaning the place out were sales, administrative and a few on-air guys who weren't into the engineering or nuts and bolts side of things, they just saw a bunch of old, antiquated stuff that needed to go. I ended up grabbing a few things with permission from the GM, but I had no place to store much, and as the old saying goes, not everything is worth keeping and you can't hang on to everything, so to the dump it all went.