To address some of the questions raised...
No, you cannot buy a region from an existing lord (unless he is seriously wounded). You can only buy lordless regions or regions with lords seriously wounded or captured. But captured removes lordship anyway...
You cannot buy a capital region or a region bordering a capital.
Militia will remain loyal to original realm and resist the new lords.
It's a minimum of 4k-5k family gold to even see option. Cost is variable upon region type , population, and loyalty/sympathies to original and bribing realms (and less so, other regional stats).
There is a risk of failure, stretching from losing some gold and escaping to losing more of your gold and being imprisoned, with option of being banished.
You cannot buy regions of your own realm.
You can set to either buy immediately or for it to resolve at turn. The turn option may be broken. The turn option, for sure, does not currently notify the realm of the briber/purchaser of the new region.
Regions join the duchy of the one who bought them, or failing that, the nearest duchy of the realm.
If anyone has any more specific questions on this or other unknown BM bits, I remain happy to answer.
