That's no great mystery, they most likely have their own Game Boy Player and disc. It's easy enough to borrow it from your own Game Boy Player to test a loose GBP to confirm it works before listing it.
And I don't follow prices, but a few might be intentionally breaking them up to sell separately. I wouldn't be surprised if some idiots would bid almost as much on a loose disc to replace one that's missing as you'd pay for a GBP with the boot disc.