See this is why I always think you're joking: You often say smart and experienced things about bannerlord, but then some times you just keep missing the point no matter how many people re-phrase it!
The problem is : The Bound cross bow CANNOT spawn into any stores EVER in any builds of the game for the last 4+ months. The same is true for many high price items. The reason they discuss the high price is because all items of high price cannot be created in a shop because prosperity in recent versions cannot EVER get high enough for the shop to make it.
I might be possible for sharpshooter to drop one, idduno, but it would probably be a damaged version.